supporting old military aircraft

This is probably not the most exciting question ever asked on here, but…..

Many airforces operate old military aircraft which have been out of production for years or decades e.g.

F-4 phantom II
F-5 Freedom Fighter
Mig-21 (all variants)
Mirage III/5/50 variants
Su-17 Fitter variants
etc.

Where do the various air forces which use such old aircraft get the spare parts from?

Did the original manufacturer (some of which don’t exist anymore) just build hundreds of thousands of spare parts?
Or do local aviation industries manufacture the spare parts under licence or otherwise?

It must be quite a feat keeping 30, 40 or 50 year old designs in the air & “good to go”.

Answers on a postcard please 😉