Sometimes if a book is pretty much MIA you can contact the publisher for a copy of the book. So for like the one you can't find, call up the publisher and just say, "Hey, I can't find this book anywhere; can I get it thorugh you guys?" I was at Barnes & Noble this weekend and they were out of a book I was looking for. Turns out that the store chain didn't even carry it anymore so they suggested I contact the publisher for a copy since if they (a large book chain) didn't have it, the odds of finding it anywhere else was slim.
Good luck with your search!