You might try CVA or Dixie Gun Works, among others. I don't know who is still in business as I have not bought a new BP weapon in over 15 years.

I have a .32 caliber "squirrel" rifle that I bought over 20 years ago, (don't remember where,) with a 36" barrel. It uses a 30 gr, FFFG charge, (pistol powder,) .310 RB w/patch and has over 2,000 fps muzzle velocity, and has excellent accuracy; it came with a double "set" trigger. (I use it for rabbit, as there are NO squirrels where I live.)