My BIL's tranny bit the dust (109K miles) a couple of weeks back. He is getting a Jasper installed as I type this. It was a $3500 cash deal at an indy to get the Jasper (indy ordered it and is installing it now).
Gator transmissions and Level 10 transmissions are the other two reputable rebuilders. Search for any of the company names on here to read up more on 'em.
I had mine rebuilt by a reputable transmission shop in the area. With the work involved in swapping a transmission, I'd prefer to put the risk on the transmission shop. If you do it yourself and it doesn't work....you'll have to do it all over again.
If you do decide to buy one used, make sure the salvage yard does the work, so if it doesn't work, they pay for the next swap, not you.