Where they're currently cheapest, I have no idea. Though I do know that kjeeper10 recently bought some. Maybe he can tell you where he found his. They're not very expensive, regardless (compared to many other Jeep items, anyway).
I got mine with my Full Traction lift about 5 months ago.
As for size, the 5100 in the Bilstein 5100's IS the size. So, just order Bilstein 5100's. Also, you'll need a TF exhaust spacer to keep your ds from hitting the exhaust due to the increased downward travel of the shocks (whether you buy the TF shocks OR the Bilsteins). The spacer is around $50, and you can install it with a helper in about 15 mins.