The problem with used tapes is you don't know the quality you are buying.
There are two separate issues I have identified with old tapes:
first, the tape might be of the batches that suffered from shedding. These tapes are practically useless and may cause problems to the decks requiring thorough cleaning after their use.
The second problem that I have seen with very old tapes that are otherwise working well is that they suffer from severe dropout. I have digitized some reels for a friend and the reels - being 25+ years old - appear to suffer from dropout, especially in the ends of the tape length.
On the other hand, new tape is not really very expensive and you get a much much better result.
I have bought a box of 20x 1200ft Quantegy 475 reels for about $50, and the audio quality is really excellent. Works well even on my low speed 1 7/8 Revox B77 with no dropout and excellent frequency response and low noise.
On the other hand, I tried a NOS BASF LPR35 tape and although it was not used and I unsealed the box, the recording quality is not at all comparable to new tape.
Same with a reel of new RMG tape that I bought for testing. Excellent audio quality and absolutely no issues with it.
I would only look into the thrift shops and ebay auctions for pre-recorded tapes with historical value, recordings that cannot be found anywhere else. Mainly a collectors excercise.
In the UK, I have found an online source at
http://www.stanleysonline.co.uk/category-59.htm
The prices appear to be similar (or lower) than ebay prices I checked.
I am sure that there must be several online sources in the US for reel tapes.