Lately I've been rethinking using the NPC2212 motors for my droid, mainly because they're almost $200.00 a piece. A cheaper option would be using electric motors off of those toy scooters, such as the Razor E-100. Fortunately this doesn't require changing anything I've already done. I can still use the same electronic speed controller, etc..
So I did some looking around and round some replacement scooter motors compatible with the Razor E-100 for less than $20.00. Even better is the website is offering free shipping for a limited time. Here's the link, http://www.partsforscooters.com/119-17?sc=2&category=77293.
Note that this motor uses a belt to drive the wheels. This may be a problem because Razor has discontinued belt drives and now uses chain drives for their current line of scooters. As a result, belt drive wheels are hard to come by. Very hard. So why did I get the belt motor instead of the chain? Mainly because a chain drive makes a lot more noise than a belt drive. For the wheels I plan to either find the wheels or build a home made wheel the belt can work on....famous last words... :)
Update: I found some generic wheels that are "compatible" with the Razor E-100, though not an exact copy of the original. But as long as it fits the belt it'll work. They're $40.00 a piece however so I'll have to take my time in acquiring them.