Best place to buy used ds?

+1 for Discogs, which is my go to online store for hard to find discs. Locally, I like to shop at Silver Platters at their huge flagship store in Seattle. They also have a website which you can shop online.

https://www.silverplatters.com/

I buy them for 3-4 bucks at my local record store.  If you have a half-price books store near you, they usually have a large collection and here at least, they are typically 3 bucks.  They also sell online.

Best bargains are charity shops, or second hand record stores in their cut out bins.  Also if libraries have sales

You have to go out and hunt,them down...there out there.

I buy a fair amount of used CD's, but very rarely online. 

When I do, I have had good luck with Ebay and Discogs.

However, I am pretty lucky to live in the LA area, and we have quite a few brick and mortar stores that I go to.