Ask a dozen guys who make tampers, and you'll get a dozen different answers.
I actually make them in a variety of ways. Some are all wood, turned on the Jet Mini. Some are all metal, turned on the South Bend 9c. Some are pure freehands, ground away on the belt grinder.
On Friday I made one to go with a pipe from processed locust, then dyed it and polished it to get an interesting effect:
Another I made on Sunday has a briar "foot" and a piece of synthetic tortoise shell on top. In htis case, the briar has a mortis about half an inch deep where the synthetic material's matching tenon is inserted and epoxied. It's been paired with a pipe that has a similar finish, and a bit of the synthetic material as an accent piece on the shank.
You can make a tamper with a metal foot - but it's not strictly necessary. I have a briar tamper that I made several years ago that I use daily. About the only hint that it's that old is the patina on the body of the wood. The foot, where it meets ash, has been blackened, but not charred, and most of that was in the first couple uses.
Guys that make tampers typically make all their own pieces. There's not really a store to buy that kind of stuff. If you do decide to use a brass or aluminum foot, be sure you use a mortis/tenon arrangement to help secure the foot, and be sure to use an epoxy that won't let go when it gets a little warm.