These are illegally made CDs and thus are illegal to sell, however you can find them in used record shops if you look hard enough. The argument is that the record companies and artists are ripped off by the sale of such material because bootlegs make outrageous money for the people who make and sell them. The trading of bootlegs is a general practice amongst fans, and is usually viewed as a harmless act unless money is charged.

"Mystery" bootlegs