+1 Pandora.

I trained an "aerobic tempo" station with a few examples of workout-rhythm music I like -- some big band, some disco (hey, I grew up with it, it's in my head), some rock, "Time Warp", some Weird Al... and then it went and found some funk and techno that fit the bill.

Just take a little while to get it partially trained at home, before you depend on it to keep you going during your workouts.