Tddeangelo,
There have been some good suggestions provided here for inexpensive targets for just general practice. However, there are a couple of things to think about if you're considering shooting some local club matches or higher level competition. Shooting bullseye targets is good, but consider that when doing so you are getting into the habit of always shooting for the center of mass. This is generally the "10" ring and the best score. However, when you encounter a Haffner, fun target, or other novelty target, the center mass of the target is generally not the highest scoring place on these non-circular, odd shaped targets.
Though these targets aren't shot at 100 yards, most off-hand rifle matches involve targets at ranges like 25 and 50 yards also. If you let your eye center your front sight in the center mass on one of these targets, your scores will suffer. It would probably be worth your while to pick-up some of these targets and get used to shooting them as well as bullseye targets. Most clubs use some of the NMLRA targets you can see at the below link.
Glad to see you prefer flintlocks, that's nature's intended ignition system. Sooner or later the "Spark Side of the Force" gets most shooters.
https://nmlrashop-org.3dcartstores.com/Targets_c_12-1-3.htmlMole Eyes
