By the time you buy the parts, you are better off buying one. In
answer to your question, it doesn't have to be a certified part. All
a transmit switch does is ground the key line on the radio.

I can't speak for all portable PTTs, but the ones I've bought actually have
2 switches - one to key the transmitter, and one that connects the
microphone (I cut the former so that passengers have to press the PTT to
talk over the intercom - cuts the noise down considerably when the cheap and
nasty squelch control keys all 4 mics at once