Justin,
Here is something I can speak to from actual experience
I get more joy out of making simple furniture then out of complete builds.
You'd be best served forming your own. Not sure where to get them but industry uses big iron balls (like in pulp mill rotating drums and such) they come in a range of sizes. Between the balls and relief carved into hardwood end grain there is not much in the way of trigger guards that cannot be formed around them. Something like a butt plate can be cut down from full size stuff more easily then it can be cast from scratch and RR pipes/entry pipes are kitchen table work. Propane/mapp oxy hobby torches are inefficient but sure do weld/braze little stuff well.
Old artillery cases are the stock you need. It works like a dream and because they are tapered a case gives you a range of thicknesses. They sell for scrap prices. The book store beside the Saint john City market has a window full for 25 bucks a piece which is less then scrap price.
If your looking for ideas for a small musket "the Trade Gun Sketch book" has a .410 NWTG build.
