A frizzen Cover, or " Hammer Stall", is an item that most people make for themselves. Its a very short evening sewing project. As has been suggested, you don't have to go out and buy a side of leather to get the material. Old shoes, ( tongues), base ball covers, leather gloves, leather wallets, purses, belts, etc. can all provide the material to make these small items. You do need to take care so you don't stab a finger or 5, but most of us are able to do that much caring for our own hides! My hammer stall is such that when I lower the cock of my flintlock, the edge bites into the leather stall to protect the edge during transport, and to absolutely prevent the stall from coming off until I cock the hammer back to full cock, from half cock. I have never had a soft leather stall cut by the flint's edge doing this.
LIke the stall shown in Roundball's photo, I have a strap or tab, made from some of the same leather, that has a slot in it, so that I can loop it around the front of the trigger guard, where it will hang down out of the way when I need to fire the gun. I have owned the gun for 28 years, with the same hammer stall on it. Still works fine. I don't even think about it much anymore, unless someone brings up the issue here on the forum.
In all the years of going to the NMLRA shoots at Friendship, the only place I have seen hammer stalls for sale was on a blanket in the primitive emcampment. Dixie probably has them, there, but I just don't remember seeing them at their booth. They have so many items hanging from the rafters overhead, I just may not have recognized them for what they are. Check the suppliers in the Links section at the top of the Index page to this forum, under Member Resources. Many have on-line catalogues, and you can find items there like hammer stalls.
From my perspective only, its hard for me to pay good money for something I can make in an hour or less, while watching TV.