The battery, depending on where in the world you are,....
Duracell Specialty 2032 Lithium Coin Batteries - Pack of 2 £3.99
Panasonic CR2032 Battery (2 pack) - Lithium Coin Cell, 3V
AND the batteries last 5+ years so it seems like a want, rather than a need to change.
The Hard Disk Drive? I DEFINITELY go with @CherylG on the make and model.
If in doubt please ask.