Radio replacement
Any qualified repair station can sell and install a radio certified for your aircraft by STC. As far as the TXPDR mode S - you will also need to have the Altimeter system certified at the same time- preferably by the same shop. I'm not sure of the reg as I have been out of GA many years but it was FAR 91.177. Manufacturers have their equipment STC'd for most of the aircraft made in the last 50 or so years.