One would assume that wherever you source other replacement parts would be a good start in looking into finding gears as well. If your existing supplier does not stock gears than I would suggest finding another source to acquire your parts.
Just because something is "metal" does not mean that it will not break. The cheap pot metal used in lower end airsoft guns is often very soft and can break rather easily.
Here is a list of some websites both in and outside of Canada that you can use for future reference to source replacement parts.