First off, welcome to Focus Fanatics!
Second, just about any ol aftermarket radio should work to play mp3's. It's the head unit that was restricting your ability to previously. I Got my stereo from crutchfield, it's really nice .it was like 120 for a really nice Sony head unit that can play mp3s and stream Bluetooth media! It even comes with an adapter to fit an aftermarket radio.hope that helps!
ROTM June '13, PNW Crew #42, SR #245, Rally Rev #02