Only use clamps if this is for live sound amplification. If you're planning to record then it is an absolute no-no to clamp the mics onto the drums. Mics are very sensitive to being moved, and when you have the mic attached to the drum, it's getting moved all the time. That noise is acceptable for live applications, but not for recording.
Clamps should be available at any local music store that has a drum or live sound section.