Over the years I have never thought of buying a mandrel to make jump
rings with. Being a rather "frugal" sort, I make them as I need
them. Although I have used brazing rod, my favorite for frugality
and easy availability is common nails. They come in a wide variety
of sizes and the mild steel is easy to work with (I've even made
oval ones), and you can't beat the price! I chuck them in a
handpiece or drill, depending on the diameter, bend the end of the
wire at 90 degrees, and slip the end between the jaws of the chuck.
They spin right up and slip right off.