What is the name of the weapon I am describing. It is similar to br knuckles.?

If all you need is a metal object to hold in your clenched fist like a 'roll of quarters', then would a length of cylindrical steel rod of about 1 inch diameter and about 4 inches long do the job? If that's all you need, you could probably get that from an ironmongers or building supplies store, either cut to length by them or by yourself with a hacksaw or similar tool. If you cut the metal to leave an inch or so projecting beyond the top and bottom of your fist then grasped, I guess you could use it like a kubotan as well.