I like Mix Meister. http://www.mixmeister.com/ They have a very reasonable and easy to use product. You purchase the songs (from Amazon or iTunes etc) and download them into the program. You can then change tempos (no chipmunk vocals), add sound effects, and blend the songs however you want. It is fun and easy to use.
I set aside a few hours a month to search for music. I usually go to the “licensed” music websites and search for a particular genre each time I do this. They usually have pre-packaged music that I will listen to a few seconds of several of the pieces to get a feel for the music. Luckily, with modern tech, I can rearrange the playlist and edit to make it fit my desires. It is time consuming, but it gets me the exact music to workout that I am looking for and it is part of my job. Easy is usually not quality.