Looking to buy/make a toe/heel shifter for my AT3. Where do I start

Some earlier Yamahas, and possibly other makes, had these as standard, so maybe look at Partzilla, CMSNL etc parts lists for these. Once you find one or two for sale, see if you can get dimensions of the spline, as well as length, as obviously you won't want it fouling anywhere.

I sold a new one a few years ago, but can't remember what it was off!