You can search specifically for a "TRA7020" but basically any 8x12mm ball bearing will fit. I say basically because there is a width dimension but off hand I don't know. It's somewhere around 4mm.
This guy has all 4 corners w/ bearings included:
https://www.ebay.com/itm/Traxxas-1-1...MAAOSw9uFW9nDp