Go on ebay and search for "vintage sci fi book lot". You can usually get a box of 'em for a buck each, and if you're patient, even $.50 each.
Your local thrift shop often has a pile of books for sale, and if you take a load of them off their hands, they'll usually give you a deal. I sometimes fill a couple shopping bags with sci fi books that way.
The fun thing about buying them by the bushel is poking through them, and discovering great sci fi you never knew existed. I enjoy the lurid artwork on the older ones, too.
