The only book actually published as "by Kilgore Trout" is 1975's Venus on the Half-Shell (a title attributed to Trout in a Vonnegut book somewhere), written by Farmer. It was out of print for many years, but was published again in 1996 by Buccaneer Books (ISBN: 0899683061), and shouldn't be too hard to find through your local independent bookseller. I thought it was hilarious when I read it in junior high so many years ago: randy satirical sci-fi just like you'd expect Vonnegut's Trout to write.