Back in the day when I was selling stuff, I purchased frizzens from both Lyman and from Thompson Center direct, as a distributor.
I would imagine the parts would still be available.
Going back some years before that, I bought parts from Log Cabin Sport Shop in Lodi, OH.
Or, if you cannot find anything from all the usual suspects, try contacting Lyman directly. If they will not sell direct, ask them for the names of their warranty repair centers. Surely, one of them would have retail parts to sell.
Fuller flints are brought in by a lot of companies. Track, Dixie Gun Works and The Gun Works are just a couple of names that come to mind. Sorry, I cannot recollect who is bringing the French amber flints in, but i hear they are pretty good, possibly better than the English ones. Hope that helps.
John