I don't know of a kit. But, if your copilot's yoke has the blank-off plate on one end, with a couple of screws in it, then its hollow and will easily accept a PTT switch.
Just pick any suitably sized SPST monentary-on switch from Aircraft Spruce, some 20ga aircraft wiring, a red (20ga) PIDG ring terminal, and maybe an appropriate connector pin for your intercom, all with the proper tooling, and your good to go. The job takes 1 to 4 hours depending on what kind of intercom you have, and how big a mess the backside of the panel is. If its really bad, your A&P might take longer to fix problems he finds.
Parts probably less than $50. Shop rates are all over the place. I recently heard as low as $90/hr, and as high as $200/hr.
All this is just a semi-educated guess, based on past experience. YMMV!
PS. If you're in a hurry to get some dual instruction, depending on how your copilot's mic jack is wired, you might be able to use one of the velcro on PTT switches like this one:
https://www.aircraftspruce.com/catalog/avpages/pttswitch.php?clickkey=8638