Best online source for music CD's

Garage sales, Discogs and thrift stores. A few years back, at a garage sale, I bought five boxes ( about 1200) of CD’s for $150. I kept about 90 sold the rest (about 1100) to a friend for $100. I was able to fill in some holes in my collection and add 90 CD’s to my collection for about 55 cents a disc.  I have also bought SACD’s and HD CD’s for next to nothing.  But if you’re looking for that specific disc or that hard to find album, Discogs has been great for me.

CD Japan, Discogs, Ebay and SDE (Special Deluxe Edition) also Rhino and don't forget your favorite band's website. 

In addition to those already mentioned add oldies.com especially for hard to find and out of print jazz and EU re-issues.  They are somewhat slow on delivery, but prices are fair and service is responsive.

@billstevenson 

 I wasn't aware of oldies.com. Thanks for mentioning it!  

Discogs and thrift stores but there's also a great independent record store not far away that has a lot of used cds.