Found a new .58 cal GM barrel at Dixie Gunworks on the scratch and dent gun rack two or three years ago. It was missing the rear sight and ramrod. They were asking $200, I offered $150, we settled on $175.
Two years ago, shortly after the. 58 cal find at Dixie, a local muzzleloader show typically draws folks from surrounding states. A vendor from the Indianapolis area had three GM barrels for sale. I purchased all three barrels, sold two of them to pay for the .54 barrel I wanted to keep.
Just a few months ago, a local auction company had an online sale due to Covid-19. I drove up and inspected three barrels. Two were T/C and one was .36 cal GM barrel, all in good used condition. I ended up with all three barrels for under $300 out the door.
The GM barrels are out there, however it seems if you are impatient or buying from online auction services your going to pay a premium. I agree with Eric, if you cannot find what your looking for have it custom made, it maybe more cost effective.