I'd guess that Adam Harper still has a few gearboxes for sale , plus a gent in Milton Keynes puts a few up on Ebay every now and then , so there are a few still out there.
I looked into getting some metal gearboxes professionally manufactured , ones that would be a direct "plug in" replacement - but was put off by the £1000 cost (per unit !)
A few of us have approached some engineer types with their own lathes, CNC's etc but nothing has emerged as yet.
I've not given up and am still exploring a few alternatives...