Probably the usual places i.e. ebay, maybe Gamestop, etc.
One option may be to buy a cheap HDD model and then replace the HDD with a higher capacity one, although this requires flashing new firmware onto specific model HDDs
Years ago I got a 20GB from GAME for a few quid.
Haven’t looked at prices, but another option may be to see if any consoles come with one, especially ‘spares or repair’ if the fault is unrelated to the HDD.