Sourcing obscure classical or jazz albums

Wanted to get advice on where to get hard-to-find classical music.   I came across a really cool piece on Youtube - two female singers with the most incredible voices -singing Le Troisieme Lecon by Francois Couperin.   I listen to mostly rock and jazz, but this is such a cool piece of music.    Tried to look up the album on good old eBay and of course they did not have it.   I suspect there are a lot of classical music albums that are just not listed on eBay.   There must be a site where hard-core classical music audiophiles go to purchase vinyl.    Does anyone have any suggestions as to where to go to find classical music albums, outside of eBay?   By the way, here is the link to this song.....very cool.  Thanks in advance for any advice!!!