You can find Trapinch in the White Forest in Pokemon White. But if you have Black, your going to have to trade with someone else to get it, or use the dream world on the computer. The dream world area it is found in is the rugged forest, and it will have the ability Sheer Force, rather than Hyper Cutter or Arena Trap.
I hope this helps you in finding a Trapinch! Flygon is a great Pokemon, especially with Sheer Force!
Edit: Oh, Flygon still keeps Levitate xD