Sometimes you can find one at a reasonable price on this site.
http://www.trackofthewolf.com/list/category.aspx/485
I started out with muzzleloader shotguns for spring gobblers with this T/C Blackmountain Mag. 12ga. If you can find a used T/C New Englander you might get a decent price. I killed this one bird with it and then went to flintlocks.
They are all so ridiculously priced I just started building my own flintlocks.
Is it a kit that you buy and assemble or much more hands on gunsmithing required?
Traditions..if they're still in business..made a fairly simple kit for percussion and flintlock shotguns and rifled muzzleloaders. You just assembled the parts and finished the stock. The prices were about as low as you could get.
Some builders sell guns in the "white"..which are assembled and you just finish the stocks, etc., the way you want. Others sell rough pre-shaped stocks and components and you have to do gunsmithing work.
I personally don't buy the kits or pre carved stocks and do a lot of gun smith work.
A good place to find what's available is this site..on the Gun Builders Bench. Just ask about what you want and I'm sure you'll get up to date info.
http://www.muzzleloadingforum.com/fusionbb/index.php?