I also would recommend Ebay, the tapes for the big names, tend to go close to the original price, but some of the tapes by lessor names, go very cheap. It takes some patience, as you frequently get outbid at the last minute. I have bought about 40 tapes on Ebay. Once you are done with them, you can resell them for close to what you spent on them.

If you buy the more costly Dance Vision tapes, you also can sell them for much more and if you resell them, even those are very cheap.