Well, that was part of my job. In the defense industry, it is referred to as "diminishing materials suppliers" (DMS). When a supplier decided to stop producing a part, I had to identify an alternative. Sometimes it was a drop-in replacement and this was just a matter of ordering samples and testing them on our boards. Sometimes we had to qualify new suppliers to build our custom parts. Sometimes I actually had to modify the circuits to fit a new part.

Generally, near the end of production the program makes a final buy of parts to warehouse for "spares and repairs". The typical threshold is twenty years of support. If the spares run out during the support lifetime, it is up to the customer how to handle it. Sometimes we can cannibalize other aircraft, sometimes we repeat the DMS process for the parts we ran out of, sometimes (if the customer wants to keep the aircraft flying for a while longer) we do a tech refresh and design new replacement modules with current components.