I would give these guys a call
https://khkgears.net
You need to tell them you are looking for a plastic 30 tooth gear...before you call you will need to know these measurements:
The diameter of the hole..(if they don't have the exact size, you can get a smaller hole and drill it out)
the distance between the outside of the hole to the tip of the gear
the thickness of the gear you have
from tip to tip across center
I would also take a couple of good pics and have them ready because they'll probably ask.
As an afterthought...if you don't want to go that route(or they don't have it), you can get a good quality epoxy and glue a circle of thin plexiglass to the wheel you broke..if you broke a tooth, that won't help you.