My copy of "Firearms of the American West 1805 - 1865" contains a picture of a 'plains-style rifle of English pattern, that was built in Philadelphia. It has a .75 cal. bore, and an octagonal barrel, looking to be about 1 1/4" across the flats, straight octagonal. 8 bore is .835". I am not aware of any rifles that large, but there were punt guns for market hunting ducks, geese and other shore birds from 4 bore (1.052" nominal) to 1.5" bore that shot upwards to 3 pounds of shot from the larger specimens.
As to availability of such a large breech plug - I know of none. 7/8x14 is the largest I am aware of in commercial plugs and that would be too small, at .875" OD. An 8 bore plug would have a 1" thread diameter I'd think. I know of none, but most of the gun makers or even a good machinst should be able to make one form a picture or diagram.