there's a record store in nashville called
phonoluxe that sells promo CDs as probably a good 25% of their product stock. you can find stuff there you never even knew existed. i've got all kinds of discs stamped with the "here there be dragons, go directly to jail, a pox upon thee" warning, letting you know that the seller doesn't have a right to sell to you. still they sell them openly and no one seems to care.
it's one of the best independent record stores left. i've pumped a lot of money into the record industry as a result of getting my hands on cheap promos and subsequently wanting to own an artist's back-catalog. it would be a shame if this practice were totally put out of business because, as the name implies, the discs are awesome promotional devices.