Joy has the right idea.

Wrecker and 'pick a part' lots are great places to get stuff to test with. I've been planning on buying a side window or two to test out my ERT-1 with so I know what it feels like should I ever need to use it to break a window. I could easily do the same with a seat belt.

If you do though please buy the parts you destroy. It's very easy to just wander around and break windows and cut seat belts in a place like that...and that's vandalism.