Make sure whatever you buy is good for the ammo you want to use...there is everything from black powder to safari loads available that will detonate an older gun. Great round though, good luck in your search.
Chiappa makes a falling block .45-70 reproduction of the Sharps rifle. They are very proud of their single-shot rifles. Around $15-$1600.
I think they also make a reproduction of the 1885 High-Wall, but it ain't cheap either.
Never had an urge for a felling block in 45/70 but do have a 45/70 trap door that is fun to shoot. Got it on the cheep a butchered rifle and converted to a carbine.