Yea, those are really common. Wal-Mart, Circuit City, Best Buy, Staples, Target, hell even some drug stores like Walgreens, CVS, and I know RiteAid has em all. They usually come with a "car kit" that includes the cassette adapter and a power adapter too so you don't wear down the batteries. Usually $10-20.
http://www.aaadv.com/Audio/Minidiscs...%20adapter.htm
http://shop.store.yahoo.com/kb-electronics/ca-706.html
http://shop.store.yahoo.com/kb-electronics/cd-330.html