if you are still looking to have it nitrided we get all of our coating done at parker trutec. I don't know if they will coat individual parts, we usually have to wait until we have a decent sized order, our last order was 2 barrels, a 1911 slide and frame, and 10 desert tactical arms bolts. The cost to have it all done is very reasonable though!

http://www.trutecind.com/trutecind/isonite.aspx