I am
guessing that your rifle uses a 1/4 x 28 thread. A Thread Check'r from Brownell's is a good investment.
Midway USA has genuine T/C nipples for sale:
T/C 1/4 x 28 Musket Cap Nipple
Midway has a few in stock, and they are actually on sale right now. Not cheap, but if you really think you'll be using these it might be worth it to get two. I kind of get the feeling T/C may have discontinued them, and they may be difficult to obtain in the future.
I don't know if Track of the Wolf has anything comparable. If they do, its probably cheaper than the T/C nipple from Midway.
I don't see anything wrong with trying musket caps as an option, as you propose, although
Grenadier's recommendation for CCI #11 Magnum caps was spot on. Those should provide the extra "heat" that you want, if you can find them. The #11 caps are in short supply right now, while a lot of dealers still have musket caps in stock. It pays to shop around. As
centershot suggested, I think it's smart to buy caps by the case, once you know what you want.
Good luck!
Notchy Bob