You can use a service and the DJ will mix up a CD for you.
John Sines does that on 32mixes.com as mentioned about.
There is another site (not to be confused with that one) http://www.32mix.net/
Power Music also has a site where you can mix your own
Have to say I just love CardioMixes.com!!!
Also there is a great app you can get on Yes Fitness Music
http://www.yesfitnessmusic.com called Tempo Magic where you can adjust the bpms of your own ipod music.
I make my own mixes with Mixmeister. (http://mixmeister.com/). Their basic program is $50 and easy to use. You can change the BPM (without any chipmunk vocals) and seamlessly blend songs. I mix formats and years and every student finds at least one song to love in each class. I am also inspired by the music and know I give a better class because I enjoy my mixes.