I have a TC Factory rifled 12 gauge slug barrel, 24" length.
It gives outstanding accuracy using 2-3/4" Federal Premium 3/4 oz solid copper Barnes Expander slugs @1900 FPS.
For a slug barrel, I'd go factory.
If you still insist on custom slug barrel, try Bullberry.
I have a rifle barrel on order with them and it's a 3-4 month wait.