New frontier armory, wild west guns, deus ex machina, gun butler, or pretty much any machine shop since all you need to take them is the barrel, and any decent machine shop could shorten the barrel and thread it 1/2-28 for you.
Gun Butler has a mill, but not a lathe currently, He and Titan have corrosponded off the forum and he will be set up shortly, When that happens he will work up a list of services along with est. price and lead time. It will be located on the Gun Butler thread after it appears. Thanks for the shout out. NYECO