The only method I know to hit all of your marks is to buy CDs and rip them yourself. Preferably buying them used locally or from eBay to make sure the "cheap" requirement stays as low as possible (to offset the effort of ripping).

If you don't care so much about Cheap, then Amazon, as Tony mentioned, probably has the best selection of Non-DRM tracks. iTunes has many and rumors speculate they'll have the majority of studios signed on for DRMless sales very soon.