Buy a test disc...
Or make one on your computer (if you have one). Freeware is available for both Mac and PC. Save your file to AIFF (Mac) or WAV (PC). Then burn your file to a CDR. You can make a disc not only for your sub to main crossover, but for testing your whole system as well. (or if you do not want to go through the motions, just buy one!)
I prefer to make my own because I can pinpoint any frequency I choose.