High-quality yet cheap synthetic "felt" insoles from the German army. Unissued military surplus. These things are made for marching but hiking they’ll also do without needing some drill sergeant yelling next to you.
Made of soft-ish synthetic felt material that forms to your feet. It's just thick and spongy enough to talk about a nominal shock absorption feature. What makes these really nice is the breathable construction without PVC or rubber layers anywhere. Just make sure the insole fits tight in the boot.
Unused Bundeswehr, a.k.a German military surplus. Nobody has stomped or sweated on these. So, a very nice deal indeed.