There a 30 inch T/C 1-66 round ball twist and a 32 inch green mountain 50 cal barrel 1-72 twist both on ebay both state they are either new or like new. Both came off a renegade so its a 1 inch barrel and will work on TC Hawkens 1 inch stock. Other than that good luck they pop up once in a while but Green mountain stopped making the drop-in barrel. hope this helps. Ohh also Oregon barrel works makes them as well but cost about 350 each.
Check with stonewallcreekoutfitters.c I think that they are making a drop-in on a custom bases. Tip Curtis also sells some a friend of mine has bought several from Tip. His address is in the back of MuzzleBlasts.
Got flinter and percussion TC barrels that have been rebored to .58 but I'm still clinging to that last unfired 1990's production GM drop-in just 'cause I'll wish I did. And well OK, because I'm a pack rat.
Getting one rebored would be a whole lot cheaper that a new drop in. I don't know about time how fast it is get one rebored. I think that Tip has some in stock and the Stone Wall Creek is probably custom make.
I look around quite a bit and don't see any drop-in barrels from Green Mountain out there anymore. I would watch the classifieds, auction sites and Ebay. I have another place to look yet and send you a PM if I find anything.
Pecatonica River has some listed on their website. I don't know if there is any left or not but if your serious about getting one I wouldn't wait to find one. It's getting pretty slim out there.
Green Mountain stopped making the drop-in barrels earlier this year. They sent their last batch to Pecatonica River a few months ago. Try them, they might have a few left. I bought the last one Green Mountain had in stock (they had one left that was missed when that last shipment was sent out, so I bought it while I had them on the phone). If you can't find one from a retailer, your only option will be shopping around for used ones.