Originally Posted by Engine

You need a cassette player/recorder, a cord that connects that to your computer (perhaps a RCA/USB adapter), and a computer that runs Audacity (which is non-proprietary i.e. free). Then play your MP3s on your computer and record them on cassette. You will need to press and hold the record button on your cassette player while your computer plays the MP3 through Audacity. Duh.

edit: I just read your OP which says that you don't want the hassle of buying a cassette deck that can play and record. So nevermind my response. But buying one of those is actually pretty easy if you have any thrift stores up there in Nowhere, USA.