Best place to buy manual box

PST is your best bet. Anything already rebuilt professionally will cost that much or more, look at what Firm Feel and Steer and Gear charge to rebuild an original box.

Anything that costs less will likely either be an unrebuilt original that you take the chance on needing a rebuild, or a eBay piece of questionable quality.

And if you buy an original box and then rebuild it, including the replacement of the worm and ball, then you’ll have spent at least as much as a new one from PST, if not more.