Actually, any switch that is rated for use in a TX works. You can get them at a Radio Shack.
It's just the wiring in, that I'm not sure of.
In any case, most 3 channel TX will have a "3P" function. I run Spektrum exclusively, and the DX3C and DX3R have a 3P function for at least four of their switches. I'm certain Airtronics and Futaba have the same (some of the "cheaper" radios, like Flysky have it, too). As a rule, the DX3C is the affordable alternative, and is a bit more robust than the DX3R. I have both, and I particularly like the DX3C's grip, too.