Breech bolts for the 39a and variations are extremely difficult to find...been looking for over a year. Did miss one, but have had no luck so far. I am looking for a bolt assy for a 1981 rifle. They do vary a bit in design, depending on when the gun was made. Pretty good chance you would have a large population of bolts that should fit, but they are just not easily available.
I can't even get a breech bolt for a 1894, not even through a gunsmith from Remington never mind a 39a that has not been made for ?? what about a decade now?
you might as well buy a lottery ticket the odds are probably pretty similar
I can't imagine that a machinist would tackle that for under $100 & even that would be a gift.
I'm thinking that you would have to find some old 39A that was severely abused to the point that it was only a parts gun for cheap. Could take years!
As mentioned before, nice ones are out there & would take less time & effort.