How long a piece do you need? Both Sparex and Vapormatic do short lengths, with enough splined bit at one end for the pto shaft to fit, and plain round for the rest of the length. Your local agricultural engineer will be able to help.
It needs to be splined for its full lenght
1 bit is 302mm and the other 2 bits I am not sure of the lengh as I dont have the plans to hand but all of them need 45 deg chamfer on ends and some 6 mm radius grove on the ends
I've also got a set of plans for that buggy (not that I'll ever do anything with them) I thought that the tractor PTO shafts were a bit crude and that the splined drive shafts were also quite expensive.
My idea was to use FWD driveshafts and CV joints from a scrapyard, I was going to stick with VW ones because thats what I know & they seem to use Lobro CV joints which have a good reputation. In terms of Driveshaft length, most of the transverse engined models have a long and a short shaft, and the in line engined models have 2 equal length shafts. Whilst the 'long' shafts will be too long they are usually tubular so lend themselves to cutting/welding to length.
The plan was to use VW hubs and bearings (breadvan shaped Polo's have a fabricated strut so the bearing housing could be cut out), Driveshafts as above, and the VW gearbox flanges machined to fit the edge products centre drive assembly.
Hope this helps as food for thought for an alternative approach