Hi RedRiver,
Sheet brass is easy to buy from Track of the Wolf, Muzzleloader Builder Supply (MBS), or any jewelry suppliers like Rio Grande. They all have websites. For small brass inlays 1/16" (0.0625") or slightly thinner (0.005") is suitable. You can get brass and sterling wire ribbon from MBS that is 0.014" and 0.008" thick, which is OK for most wire work but on the thick side. I prefer to buy brass and silver sheet and cut wire ribbons with shears. I show how to do this in my thread on building a decorated Kibler. I prefer 0.005-0.006" thick sheet for wire ribbon. Unfortunately, it is getting harder to find from the major jewelry suppliers but companies like Surepure Chemetals sells it (up to 50' lengths in fact!). It is best if the ribbon is dead soft for most wire work, which means it is completely annealed and has little springiness. However, when I do long straight borders, I prefer half hard wire, which has some resistance to bending because it keeps straight. Finally, I don't always choose sterlng silver and often use fine silver ribbon because it tarnishes much more slowly. The higher content of copper in sterling is the reason. I am not sure what was used mostly on original guns although most British work was likely sterling.
dave
