The Baums are still available, along with several of the others. The Wizard and Land are available from countless publishers. If you are only interested in first and older editions, the search becomes exponentially harder. Used book stores, flea markets and tag sales are all good places to check. There are several dealers who specialize in Oz and related works.
Auctions are also very useful.
A major feature of IWOC conventions is an auction, which is followed by dealers setting up their tables. All are currently available and can be ordered from the club. Yes, with more being published constantly. There are even other stories by the above authors. As to the other authors, Thompson wrote several short stories.