I have had varying luck at the following sites finding books.

http://www.addall.com/

http://www.allbookstores.com/

http://www.bestwebbuys.com/

http://www.pricefarmer.com/cgi-bin/farm

http://www.bookfinder.com/


For more obscure and antique books, you might have luck at:

http://www.biblio.com/

http://www.abebooks.com/?AID=7134924&PID=265119

http://www.powells.com/

http://www.bookcloseouts.com/bc/home.asp

I hope that helps.

Tom S.