You can buy flash guards from The Log Cabin Shop or Dixie Gun Works.
If you want an "authintic" nipple protector.I would suggest Fall Creek Sutlers, or The Blockade Runner. The proctors are made of lead, attached to a small chain that attaches to your trigger guard. cost about $10.00
In regards to the flash guards, I'm not certain if they are made for musket sized cones,but could most certainly be drilled out to accomadate the larger cones.
Wendell