Advertise on things like GunBoards, and keep an eye on the Germanicon sites and such. Hermann Historica had the pieces of a bolt a couple of auctions ago as memory serves me. My T-Gewehr has the original bolt with it.

Depending on how much the fellow is asking for his T-Gewehr, it would still be worthwhile picking it up, as they are very rare nowadays. They were large and heavy and therefore not popular to bring back as war booty (hmmm, trench mortar, T-Gewehr, or Luger, which should I put in my kit bag?)

If you wish to have one roughly made up to put in the rifle, I can take exact measurements of mine for fabrication. It would be hellishly expensive to make a functioning one, though, but since no one in his or her right mind would actually want to fire one of these things, even if you could get the ammunition, it's probably somewhat moot. I've seen footage of these in action--they were crewed by two, one ammo carrier and one shooter. They would switch off after a couple/few shots, as the recoil could break collar bones, and the concussion left bleeding noses and eardrums. Bigger is not necessarily better!

Cheers

Ed