My advice - Get stuck in and rebuild your existing gearbox yourself!
I looked bought a secondhand Series gearbox from a specialist, got it home, removed the drain plug and out came a load of bearings. He took it back. I looked at several other gearboxes in his spares vehicles but they all had bearings/ iron filings etc in the drain plug. My advice is don't bother wasting time and money buying secondhand gearboxes that have not been overhauled. As long as all the gears are working in your box and especially if the synchromeshes are working well, I would recommend stripping yours down, replacing the broken parts (probably just a bearing costing less than £10), all gaskets and seals, that way you will probably spend less money and waste less time than you would buying a crap secondhand gearbox and it won't leak - for a while anyway! If you're competent enough to swap a gearbox then I can't see that you should have any major problems rebuilding a series gearbox. The Series III Parts catalogue will be very helpful for finding part numbers.