This whole idea of yours doesn't seem fully thought out. So you kind of want a "1860s reproduction" to fire off blanks in the woods with your friends who will be bringing modern firearms? Before you sink the kind of money we are talking about here into anything like a reproduction musket, I would contact a local Civil War reenactment group, see if you can borrow some gear, get some training and "try before you buy." Chances are fairly good you will find the experience including loading and firing quite a thrill. You want to be able to know a little bit about what you are doing especially safety measures before making any major purchases and heading off to the woods with your friends, though. Also as a general rule, "blank cartridges" are only sold at some larger Civil War events. Black Powder being classified as Hazmat, it requires separate and expensive shipping so you can't just buy a few blank cartridges on-line and have them mailed to you. Generally most of us have to buy black powder in one lb cans once you find a local source that sells it, some means of measuring it out as well as paper cartridge tubes to make our own blank rounds.
Since you broadly state you reside in the "United States of America" be aware that some of those states contain municipalities with ordinances regulating where, when and under what circumstances BP firearms can be discharged, like on an outdoor rifle range. Some states like NJ, you can't even buy them. Once fired, black powder firearms require very thorough cleaning. If not done quickly and properly the residual crud will quickly harden and corrode the metal parts. There is a lot more to all this than you seem to think.