You probably can't get one in your budget, but an R154 is the obvious choice, as that was what the Supra manual turbo came with from the factory, - you'd could fit it with factory parts!
They are tough as nails (originally came from a coach.. Toyota Coaster). Failing that, you might have better luck finding a W58 (Supra N/A, but later used in the JXA-80 N/A and a bit stronger).
Most of the Tuned Lexus V8 people are running BMW Getrag 6 Speeds these days, they are a lot more common than the "proper" Toyota part, a similar shape, strong and cheap. Obviously you'll require fab work.