I like the Dance Mix Workout Albums available on iTunes. They're grouped according to beats-per-minute, so you just choose the album that's in the range of the exercise you want to do.
Dance Mix Workout Music 5 (136-146BPM Music for Fast Walking, Jogging, Cardio)
70's & 80's Dance Workout Music 2 (130-132BPM Music for Walking, Cardio, Strength Training)
On the downside they rarely feature the original vocal artists. On the upside, they're relatively inexpensive.
If you already have music you like and you just need to figure out if the beat is good for the workout you want to do, you can try using the MixMeister BPM Analyzer
. It's a free tool that can figure out the BPM of any mp3 in matter of seconds. It's not fool proof, but it's accurate often enough to be a huge timesaver.