I’m pretty sure I got mine at HLJ.com.
I own all of the motors too, and I use all of them, depending on where I’m driving. You don’t want a really fast motor on a track, because you’ll shoot straight through the turns and “gas mileage” goes way down. If you’re in an open area and not racing (other than drag), you’ll be more into using the higher speed motors.
But I’m constantly switching motors depending on the environment :D, so you may want to think about that too.
A good motor/gear assortment is GREAT, if you can find a track with a lap counter. It can be a LOT of fun TESTING and finding the quickest combination! (as a hint, you’ll probably notice it won’t be the fastest motor OR gear that gets the best time)