Aftermarket batteries for Gorgon Grom/Quake Grom??

Has anyone found aftermarket batteries that fit inside the Gorgon Grom?

The stock 1400mah 2S Spektrum battery doesn’t last long enough and I could like a a couple of extra spares, but I don’t really want to pay £35 each for the Spektrum batteries.

Batteries larger than 1400mah would be good!

Thanks!