Run a search for Robert Anton Wilson's name on Amazon and you'll get a bunch of hits for books that are still in print and still available.
But it also can be fun to shop, or at least browse, at the Web site of Wilson's main publisher in later life, New Falcon Publications. The publisher has put out or reprinted many of Wilson's books, enjoyed a good relationship with Wilson when he was alive, and since his death has worked to keep his books in print. The site features an online catalog and often has specials on certain titles.
There was some kind of schism in the New Falcon world a couple of years ago, and so there is now a rival company called Original Falcon Press. I have no obligation to take sides, and I like them both. Original Falcon is the publisher of the new Robert Anton Wilson audiobook I mentioned recently. I bought it from iTunes and have enjoyed listening to it.