By and large, you don’t. When the radio was in production or at least still supported, you still had to send it to TAC, since it was developed in Rochester (military secrets shhhhhhhh…). There were never any third party repair shops and parts were limited to things like antennas, knob caps, and belt clips.
Things regarding service seem to maybe be changing though. It’s not uncommon for a company to hold service near and dear until it’s outdated ‘enough’ to start getting rid of service related parts.
New London Tech now holds most spares and who knows, perhaps will offer service in the future… whether they offer service to individuals like most of us is certainly unknown, but I can see them supporting municipalities that have quantities of radios before offering their services to hobbyists.
Bottom line for a that you should not buy the radio at this time if having a clean path to service is important to you.
-B