eBay->title search->sort price low to high-> usually results in under $5 free shipping for almost any oldish book. Also for some reason the sellers on eBay are almost always cheaper (sometimes significantly) than their Alibris/Abe/Amazon storefront counterparts. Same with Thriftbooks, cheaper on their eBay storefront than direct, especially since TB raised their free shipping minimum...

bookfinder.com is what you're looking for