They are all manufactured by the same people. However, there are a few different distributors to buy from.

My suggestion, based on what I've read, is to buy it from the Ebay seller found in my signature (click on the red text next to sprint booster).

This seller is very reputable, has not had any negative feedback with those people on this board who have purchased from him, and gets his product to the US within 6-10 days (and it's coming from Hong Kong). The only delay that he may have is if he does not have them in stock.

I purchased mine used, so I didn't go through a distributor.

If you have the SRT, you can use either the 451A or 451A2 model. The A2 is supposedly less 'aggressive' than the 451A. I have the 451A, and don't seem to think I would want it less aggressive.

SQ