If you can get to Dixon's, Log Cabin Shop, Dixie, Ft. Chambers, etc. try that first. Explain that you are new and looking for a start and they will try to help you. The people working there know what they are doing and will send you in the right direction. They also may help fit you for the gun as to length of pull, eye dominance, etc. It may not be a perfect fit, but it will be better than just guessing online.
If you can't do that, an alternative is to get a friend who knows flintlock rifles to go with you to a non-specialty shop and look at the flintlock rifles and help you pick one out. Again, this should be someone who can be your guardrails from going off the road, so to speak. You will have to cast about to find a flintlock at a regular gun shop, but try your local area and work outward, depending on how far you want to drive.
If neither of those can be done, look at any newly made guns from known makers sold by a reputable shop or dealer (e.g., if you can find a Pedersoli Kentucky at a Cabela's or something of that sort). Stock is spotty but at least you have some protection of a reputable store and some kind of new warranty.
The last thing I would do as a new person is to buy a used gun online from someone on Gunbroker, or on broker website, pawn shop, etc. This is certainly something you can do as you learn more what to look for and what fits you. When you buy a used flinter you buy three things: the bore quality, the lock, and the prior owner's maintenance habits. There are many good buys out there in this category, but as a first-timer, I'd stay on the more beaten path. You can get burned in this realm as well, and many of us have had that happen at one point or another. If you start collecting these guns and shooting them a lot, then you can go this route later.
