I regularly shoot a replica 1853 Enfield caplock rifle-musket (.58 cal) and a replica 1756 long land pattern British musket which is a smooth bore flintlock (.75 cal).
Some things you need to be aware of:
A smooth bore musket always fires curve balls. The ball is going to randomly hit one side of the barrel or the other on the way out. It will go straight for maybe 50 to 75 yards. After that, which way it goes is going to be anyone’s guess. It used to be said that you could stand 200 yards away from a single musketeer and not fear being shot by him. There are generally two sizes of round balls available for any given caliber of musket. The tighter fitting one (closer to the bore size of the barrel) will be a bit more accurate, but will also be harder to load after the barrel starts to get fouled by powder. If you are hunting where you’ll only take a few shots with plenty of time for cleaning the barrel in between, you’ll want the tighter fitting ball. If you are plunking at targets you’ll want the looser fitting ball or you’ll spend a lot more time cleaning the barrel so that it doesn’t get too hard to load.
Rifles are more accurate, and can fire either a round ball or a Minie ball, which despite its name isn’t a ball at all. The Minie ball is actually a conical bullet with an expanding skirt. The expanding skirt means that you can make the round small enough that it can fit down the barrel even after you’ve fired a few shots, and when fired the skirt will expand and grip the rifling and give the round a good spin, which makes it more stable in flight and more accurate. A rifle with a round ball is accurate to about 200 yards. You can double that with a Minie ball.
Smaller caliber muskets like Pennsylvania and Kentucky rifles will usually fire round balls. Minie balls are typically only available in military calibers, like .58 for Springfield and Enflield muskets.
You have two choices for the firing mechanism, flintlock or caplock. Flintlock is older, and is just a piece of flint that strikes the steel frizzen, making a spark that ignites powder in the pan. This causes fire to go through the hole into the barrel and ignites the main charge. There’s a bit of a delay between the pan going off and the main charge going off, and you also get a lot more flash and smoke in your face. In reality the delay isn’t that big (though it is just long enough that some people flinch and miss their target) and the flash and smoke really isn’t that bad, but when it’s right there in your face it takes a little getting used to. Flintlocks are much more prone to misfires. If it’s damp out, they won’t fire. If you don’t take care of your flint, they won’t fire. They are a bit picky, but if you maintain your musket and your flint well, they are fairly reliable (except when it’s wet out).
A misfire isn’t harmful or anything. It’s just annoying when you pull the trigger and all you get is a click. The important thing is to stand still for a good 30 seconds with the musket still pointing at the target, because you could have a small spark that will go off when you’re not expecting it. If nothing happens, just pull back the cock and try again.
Caplocks are a lot more reliable and a lot more immune to weather. Instead of flint, you have a percussion cap sitting over the musket’s nipple, which leads to a hole in the side of the barrel. The hammer comes down and smacks the cap, and it makes flame that goes into the barrel and ignites the main charge. If a caplock gets stubborn and won’t fire, you can unscrew the nipple and put a tiny amount of powder under it, which will usually get the main charge to go. If all else fails, they do make stuck ball removers which screw into the barrel where the nipple goes and can force out a non-firing load.
Worst case, you pour water down the barrel (to make sure the powder won’t go off) and use a ball puller on the end of your ramrod to get out a stuck round.
Cleaning muskets is a pain in the ass. I kinda enjoy it (it’s part of the whole experience) but the cleaning is what stops a lot of people from getting into black powder weapons. Unlike a modern weapon, you have to clean a musket every time you use it. Black powder absorbs water out of the air and combines it with sulfur in the powder to create sulfuric acid. If you don’t clean your musket properly, the acid will eat out the inside of the barrel and ruin it.
You’ve got basically two types of muskets available, military and non-military types. Your military types are things like the Springfield and Enfield muskets. Your non-military types are things like Pennsylvania or Kentucky rifles.
I live near Gettysburg, and they have many shops selling muskets (mostly Civil War era reproduction caplocks).
Dixie Gun Works (already mentioned) is another source for muskets.
I got my flintlock from Middlesex Village Trading Company. My experience with them is just one single musket, but it’s of good quality and they are generally less expensive than Dixie Gun Works. I went with them though because they were the only ones selling the early (longer) model British infantry musket. Dixie and the other places only sell the later version, which is shorter. They have a much smaller selection than Dixie, but their muskets are a bit more unique. They have Spanish muskets and fowlers and guns that you don’t see at the other places.
Track of the Wolf is another vendor. I’ve only ever ordered ammunition and accessories from them so can’t speak to the quality of their muskets. From what I’ve seen on their web site they sell mostly real antiques, but they do have reproductions from time to time.