If you have a .308 scout with a 20" barrel and you're okay with a 16.5" barrel, then measure your barrel at 16.5" and see if it meets spec. for being threaded. Chances are it will.

Aside from that, FeeBay is the place to find affordable take-offs and many of them are already threaded these days.

If you have one built, it's going to run you at least $300-350 or more for a threaded barrel.