OK; do you know whether the controller is designed to operate wirelessly (connection to a standard RC control trigger), or via a wired throttle? I ask because I don't know of anywhere that specifically sells RC skateboard controllers, but I know that you can probably rig something up. If you want a throttle that's physically connected to the controller, then you should be able to slap in something like
this for $50 or so, then grab a three-wire potentiometer or Hall throttle and plug it into the controller for $10-15.
If you want radio control, then things might start getting more complex and expensive. Most of the standard RC ESCs (capable of taking direct RC throttle pulses) that I'm aware of are designed for three-phase brushless motors (which are
very different from what you have and use very different controllers), and the only products that I'm aware of that will do that for a brushed DC motor are moderately specialized controllers intended for use in small robots; you can have a look at offerings in that domain over
here, but they tend to run decidedly more expensive than the cheaper wired small-EV brushed controllers for the same power-handling capabilities.
Or you could just upgrade to brushless power and LiPo or LiFePO4 batteries
On a more serious note, though, if you want better range and battery durability for much less weight (and have the money to spend) I really would recommend upgrading your batteries from the stock SLAs to some form of lithium chemistry.
