If by 6 speed you mean a Getrag V160, those are pretty much unobtainable. If you mean a T56 Magnum, that is definitely a possibility... you can even get a sequential if you're so inclined. Minor mods to make it fit.
Prices are what they are for the V160 because new transmissions or new parts for them no longer exist. As for the T56, I was referring to mating it to a 2JZ - see Lexus IS300 T56 Magnum-F swap kit (GR700-GR900-GR1000-GR1000F) 2JZ Altezza TR6060. If the V160 in my race car called it quits, that is the route I'd go (for reliability with a sub-400 hp car).I am just astonished how overpriced the getrag is. When I had my soarer a decent rhd supra was 10K. i have had a hand in a few 7mgte swaps and while i love the LS +t56, i would stay with a 1j and be happy with a reliable sub 400 hp car.