Jesse, just call them. If you continue to grind the gears you will damage the selector rings. This does not render them useless, but will result in notchier shifting. As Les said, they are not only hard to get but expensive also.
Dan, to change the gears you need an idler shaft and 1,2,and 3rd gears. All other parts are interchangeable. I have no idea if they are available or how much they'd cost. It certainly can be done if you can source the parts.