All of these manga are published by Shonen Magazine Comics and Kodansha (Kodansha handles all of CLAMP's manga). These editions are all softcover.
They're all in Japanese language and retail at 410円 (Yen), which is roughly $3.50 USD. But if you find them for retail domestically, you can probably expect to add up to $3 or $6 to make up for the shipping.
Some recommended and reliable merchants...
Kinokuniya - Shannon's first choice!! - Kinokuniya is a large Japanese "import" store (their U.S. locations, anyway), that you basically can't get any more authentic than. (I say this because every time I go to their east coast store, all of their employees are Japanese!) Kinokuniya is thus GREAT with getting new releases to North America within a week of their release. So if you're looking for the latest TSUBASA RESERVoir CHRoNiCLE, make sure you use their booksearch, and then you may choose what store you want your shipment to come from. But remember to use the term "ツバサ" to search! ("Tsubasa" in katakana, for those curious!) Kinokuniya only accepts credit cards, or instore order pick-up.
Akadot Retail - A California-based animanga store with reasonable prices and shipping! I bought all of my CLAMP no Kiseki from this retailer. They're great and really friendly to boot (don't be afraid to e-mail them; they're really helpful). Their newsletter is also quite amusing. =P Akadot Retail accepts Paypal and credit card payment.
Animaxis - A Japan-based animanga store with every bit of Tsubasa merchandise you could ever imagine. If it's rare, you'll want to look at Animaxis. But be sure you buy from bulk from this store... the shipping can be pretty awful until you reach the amount for their scrumptious 20% shipping discount. Unfortunately, this retailer only accepts credit cards, and not check, money order, nor Paypal.
Volume 01 (Chapitres 1 - 5)
Release Date: August 12, 2003
Summary: Syaoran and Sakura live in the Clow Kingdom. The two are strongly enamored with each other beyond their difference in social rank. But an event at the Clow Ruins suddenly caused Sakura to lose her heart in the form of her "tsubasa", resulting in her loss of memory. There is only one way to save Sakura and that is to collect all of Sakura's feathers that have scattered into parallel worlds!!
Volume 02 (Chapitres 6 - 13)
Release Date: October 17, 2003
Summary: In which of the Kudan is Sakura's memory "feather" installed!? In the Hansin Republic, Syaoran & company search for the "Wings of Sakura's memory". There, they find out that in this country, people have a spirit-like weapon called "Kudan" - parasites on each person - and that "Kudan" is the way to battle with each other. And because Syaoran possesses a special Kudan, enemies appear one after another to attack him... but despite the fact that they are not used to the system of Kudan, Kurogane and Fay are showing their power at their max! And in order to rescue Sakura's "Wing" that was trapped into one of the Kudan, Syaoran has to... Read on, and find out the "destined battle between Syaoran VS. Shougo"!
Volume 03 (Chapitres 14 - 21)
Release Date: December 17, 2003
Summary: "The Country of the Secret Arts is the next world!" Having taken back Sakura's feather and parted ways with Shougo and Masayoshi, Syaoran and the others find themselves in the country of Koriyo, the former Country of the Secret Arts, now a nation ruled by an oppressive dictator. Their mission is clear: to topple the iron-fisted ruler and take back Sakura's feather!
Volume 04 (Chapitres 22 - 28)
Release Date: February 17, 2004
Summary: The battle between Syaoran and the dictator of the country of Koriyo over Sakura's feature takes a turn for the unexpected. At the end of the struggle, Sakura's feather is returned to her. She is able to regain some of her memories, but so much is still left unknown. Putting chaotic Koriyo behind them, the companions head to the Country of Mist, where their quest continues.
Volume 05 (Chapitres 29 - 35)
Release Date: April 16, 2004
Summary: Arriving in the country of Jade in search of another of Sakura's feathers, Syaoran and friends uncover a disturbing legend about a golden-haired princess who kidnaps children. Sakura spots this mysterious princess, herself disappearing shortly after. Now it's up to Syaoran to bring Sakura back and save the children!
Volume 06 (Chapitres 36 - 42)
Release Date: June 17, 2004
Summary: Outo country becomes twisted by war... Who is the mysterious Demon Child that holds the key to everything? Hoping to gather information, Syaoran and Kurogane become oni hunters while Sakura and Fay work at a cafe. Syaoran soon learns of a mysterious change taking place inside the oni.
Volume 07 (Chapitres 43 - 49)
Release Date: August 17, 2004
Summary: Syaoran embarks on a dangerous quest to discover whether or not his mentor has become one of a new breed of oni... Meanwhile the secret of Sakura's nation is revealed, as is the true identity of Inoichi, strongest of the demons!
Volume 08 (Chapitres 50 - 57 + Bonus)
Release Date: November 17, 2004
Summary: Leaving Outo, Syaoran and friends end up in a mysterious beast kingdom. Syaoran soon becomes kidnapped by the animals, destined to become a living sacrifice for their demon lord! Determined to both get back Sakura's feather and discover the true identity of the mysterious lord, Syaoran journeys deep into the forest...
Volume 09 (Chapitres 58 - 65)
Release Date: February 17, 2005
Summary: While travelling in the country of Shara, Syaoran and Sakura become separated from Kurogane and Fay. Mysterious circumstances result in Syaoran and Sakura being transported to another realm!
Volume 10 (Chapitres 66 - 73 + Bonus)
Release Date: May 17, 2005
Summary: The battle between Ashura and Yasha comes to a dramatic conclusion, with Syaoran standing by while the two kings battle over the fate of the nation.
Volume 11 (Chapitres 74 - 82)
Release Date: August 17, 2005
Summary: Syaoran and friends become involved in the Piffle World Dragonfly Race, competing for the grand prize: one of Sakura's feathers!
Volume 12 (Chapitres 83 - 90 + Bonus)
Release Date: October 17, 2005
Summary: "Don't turn your back on life!" Despite outside interference, Syaoran and friends manage to pass the Dragonfly Race preliminaries and enter the finals. With a criminal still at large, it's set to be a dangerous race for the remaining twenty competitors!
Volume 13 (Chapitres 91 - 99)
Release Date: February 16, 2006
Summary: The heroes' latest destination is a country rich with magic and mystical libraries. A chance encounter with an unusual book leads to Syaoran experiencing the memories of a young Kurogane with a tragic past.
Volume 14 (Chapitres 100 - 108)
Release Date: April 17, 2006
Summary: After coming across the Book of Memories, Syaoran learns Kurogane's past and also discovers one of Sakura's feathers. Unable to recover the feather by legal means, Syaoran is forced to infiltrate the Central Library and attempt to steal it. Unforeseen security measures get the best of him, however, and he finds himself trapped inside!