Many of the suppliers sell inlays. Try Crazy Crow. Even Tandy leather offers a lot of options.
You can also search the internet for suppliers of jewelry products, like wire, for inlays. The NMLRA offers courses in May, each year, at a college in Kentucky, where you can learn how to do wire inlaying. And there are books on the subject, altho I can't think off-hand of one to recommend. Use your inter-library service at the local library to get these books, and read them, before you buy any of them. Many hobby stores now sell plate metals- brass, "silver" ( usually german silver, but real siver is also available if you can afford it.), and steel are there. Check Brownell's catalog, too. Wire, and rods are also sold. I bought a piece of brazing rod at a local machine shop, to use to put pins in my ramrod ferrules, and to hold antler handles on knives I made. I still have a piece of it in my range box. Its small enough in diameter to use to drift out small pins, but soft enough to not mar the pins or other surfaces If I slip.
Good luck. :thumbsup: