Parts: Miscellaneous

BSDMicroRC is a supplier for indoor flight items and has pletny of parts that can be used in monorails.

BalsaProducts is another.

HomeFly also.

Didel makes small motors and gears and mostly sells through one of the above listed suppliers.

Walthers is probably the largest supplier of model railroad items

Tower Hobbies sells radio control items and model railroad items

Northwest Short Line makes gears and gearboxes that look like jewelry (to me anyway).

 

For the hardcore, Koichi Tanaka makes the tiniest RC and IR gear I have ever seen. Unless your Japanese is better than mine you might want to try this search term *indoor airplane world e-shop* and let Google translate the web site. Tanaka-san is a great guy and is very helpful.