Traxxas have to be the easiest brand to buy parts for. Most of my RCs are Traxxas for that reason.
I also have lots of Axial, one Arrma, Hobao, GMade, Tamiya, Absima, Cross RC, HPI, Tekno, MST, RC4WD and some generic chinese things.
I only run the Traxxas though as parts for the Horizon brands are a pain to get at the moment and the rest of them aren't much easier.