I was lucky.
Stopped at a yard sale in the country a while back and while looking through the junk pile (all the stuff from the cellar) I found a couple muzzle loaders.
1 was an old knight inline, rusted solid. The other was a CVA Kentucky Rifle Flintlock that was pretty rough and rusted also. The actions on both were rusted so badly that they were frozen.
I dickered with the guy there (they were in the basement when he bought the place) and finally settled on $35. Maybe I could get one of them to work.
Brought them home and soaked them in solvent for a few days and was finally able to disassemble the knight. Took the CVA apart and poured solvent down the barrel, nothing came out the hole, rusted tight. I also put the lock parts in a coffee can of solvent.
A couple days later, I began working brushes and jags in the CVA barrel and finally got it cleaned out. I also was able to scrub the lock assembly enough to get it working. Only needed a new main spring as the old one was too weak. The stock was very rough, but I was looking to get a shooter, not a wall hanger.
After a lot of elbow grease, I had 2 working (but not real pretty) rifles.
I gave the Knight to my neighbors boy, who did a great job re-bluing it, for a first BP Rifle.
After finding a sight to fit the CVA, I took it to the range (o.k., to the corral behind the barn) and test fired it at a fence post with some balls I had cast. IT FIRED! Imagine that. Later, I sighted it in and it gives 2-3" groups @ 50 yards with 75gr. fffg and a patched ball.
Not bad for a $35 garage sale gun, although I wouldn't have minded paying 3X as much for one without the rust.
Now I'm trying to find another project or parts gun so my daughter can have her own flintlock (and give mine back).
Watch those sales, you never know.