In addition to what others have mentioned, you can try the stuff that Pavo
makes, The ICube, The ADB-I/O (if you're on a system that uses ADB), or The
Peavey PC-1600. Many electric drum controllers can also accept voltage and
turn that into some kind of midi signal. You may also be able to use the
parallel port on your computer directly if it has one.
http://www.pavo.com/
http://www.infusionsystems.com/
http://www.bzzzzzz.com/BeeHive/ADB_IO/ADBIO_Intro.html
http://www.peavey.com/kba/archive/pc1600/index.html
Issac