Unfortunately don't know many companies that sell them. Pfitzner Performance Gearbox (PPG) do the saloon car gearboxes and that. For the 4 speed (M20/21) have the option of either straight cut gears with synchro or a dogbox (the more expensive option). Just had a T5 built with their gear, cost a **** load more but handle 500hp nicely.
When you start talking racing gear, you won't get much cheaper.