A few years ago, Airfix produced a kit of a V8 internal combustion engine. But has anyone produced a kit of a manual gearbox? Apparently Meccano did many years ago, but I imagine that they're very rare and expensive these days.
I don't know what you are planning but would a real gearbox out of something small thats made of alloy be any good ? It wouldn't be that difficult to cut away areas of the casing so you could see the gears inside. I can't be very specific about a particular car you could find one from but if you consider the size of a Triumph Herald/Marina gearbox once you have removed the bellhousing and tailshaft extension they aren't huge.