Air can leak from:
Bellows
Bellows gaskets (where they join on to the ends)
Palettes (those are the pads that uncover holes when you press a key)
Holes in the woodwork - if something like a strap bracket was screwed in and later removed and the screw holes go right through. It does happen!

Bellows leaks may be patchable with tape.
Gaskets are easy to replace.
Palette leaks can happen because the leather facing has deformed or hardened, or because something has moved so the palette doesn't cover the hole properly.

Often it's a combination of a bit of everything.

Buy any replacement bits you need from Charlie Marshall if you're in the UK.