Breed's absolutely right. Be sensible, be original and be creative. Keep within creative commons.
My personal 3 favourite breaks for dnb:
- The Think Break
- The Hot Pants Break
- The Amen Break
I'm sure you know of these, but they are fantastic to sample. I can send you them in mp3 form if you'd like, just PM me your email address.
Musicradar is fantastic for more royalty free samples, and they cover everything from vocals to guitars to synths to ketchup bottles (no joke). There are literally thousands of samples/ loops there, I highly recommend them.
Go slu sum paigonz blud