Inverter failure in 2003 prius

There were two kinds of 12V used in Gen1's. Small battery with pencil post terminals and a larger Panasonic unit with normal sized terminals which used a different battery tray. The latter was often "upgraded" by the dealer after the first battery died. Easiest option is to buy a new battery from the dealer, but they can be expen$ive.

Look at what you have in your car, see if you can find something similar at your FLAPS then make it work.

There is no "direct fit" aftermarket option unless you buy the battery kit from Optima battery and installation kit for Prius 2001 to 2003 and change out the terminals and possibly the mounting equipment.