Daniel,
I would have to search around a bit for 12mm Thomson 60 Case shafting. It is case hardened on the polished surface to rc60. It can be drilled and tapped on the ends because the case hardening isn't that deep. You could also anneal the ends for threading and thread a nut on one end, weld it in place, and thread the other end after cutting to length. Probably the better option is threading 12mm X 1.50 RH, a standard lug nut thread, then source a castle or similar nut to secure the axle. I believe the shortest Thomson shafting is sold in 24" lengths. Probably more expensive than the Fastenal route.
Hope this helps,
SAT
ps
If I remember right, Fastenal used to supply 12mm X 340mm bolts, threaded 12 X 1.50 RH