you'll find out quick that no shop or builder can do a transmission that's worth buying.
every built trans (maybe not the new stuff that Jack's is coming out with) that i've ever seen, still used Mitsubishi gears, which will break. sure, it's got double synchros, billet shift forks, and a different ratio, etc, but you'll still break the same parts you would in a stock transmission. the next step up is a dogbox, then you break that too, and it costs STUPID money to replace parts in that.
IMHO, learn how to do it yourself, or get a friend who knows. then, stockpile transmissions and parts, put them together, blow em up, and don't pay for anything besides parts. again, this is my .02 cents about the ordeal. i also build all my own transmissions (for all the above reasons)