you can buy b&m ones from most auto parts retailers (autobarn, supercheap, repco,etc..) some of these places might also be able to order the transgo one....which from what i understand is the better of the two.
ive a got a transgo stage ll in my trans and i love it, i recommend it to anyone who enjoys "real driving".
other than that any transmission shops would be able to get anything you want, along with ebay, there's always some shift kits on there. they usually cost around $200 for the kit (transgo).