A trigger is just a piezoelectric sensor/speaker. You can trigger it with compression, or even with motion. For instance, you could get a kick drum pedal, mount it to a wooden contraption and have the beater smack against a piezoelectric device. It's best if you sandwich it in some foam to protect it, and give a difference between a light and hard hit.
For cymbals you can just mount the piezo to a plastic practice cymbal.
The Alesis DM5 has been around for years. They're probably really cheap on ebay. Radio Shack used to sell piezos. You can also buy replacement piezos from Roland.