If you're serious enough about running to worry about your cadence, you might consider getting a watch and some associated hardware to help you measure things more precisely. That'll keep you from having to listen to the same five songs over and over, and might also help you find a more natural cadence that is close to "ideal" without necessarily hitting every beat. In other words, a cadence of 93 might feel more comfortable for you, but will eventually bring you off-beat if you stick with 90 BPM music.
DC Rainmaker talks a bit about cadence in his review of the Garmin ANT+ foot pod, which is what I've personally used for the last two years without any complaints whatever. I use it with the reasonably-priced FR60, though any ANT+-compatible watch will do. Polar also makes a line of foot pods, though I can't vouch for those.