I've used Silencer Central
From their web page:
"Silencer Central offers a unique barrel threading service where you can ship the gun directly to us or drop it directly at HQ. Simply strip your rifle down to a barreled action for easy handling and we’ll do the rest! No matter if it is a common hunting rifle or a custom match grade piece, we’re ready to take on your barrel threading project, and give it all the attention to detail and care that Silencer Central is famous for."
I think I paid about $150 when it was all said and done. (You don't need to buy a Silencer from them, this is a stand alone service that they offer).
