Ratha's Creature - $12.95
Clan Ground - $2.95 (Impossible!)
Ratha and Thistle-Chaser - $14.95
Ratha's Challenge - $16.95
Not a Thistle-Chaser series book, but a must-read from the same author:
People of the Sky - $3.95
Hm. Poking about the site more, the links all seem to lead you back to Amazon. They're available there, but at higher prices.
And while on this topic, I still need to get the Clare Bell section of Catlove up. Lazy me just doesn't want to deal with scanning. And, uh, writing. And HTMLing up a new page or six. Maybe this weekend...