If you have trouble finding the exact type, or don't like the price, there is a handy hack that I have used. A standard 9v compact alakaline cell consists of 6 x AAAA cells spot-welded head to toe in sided that plastic rectangular package. You can peel of the package and carefully separate the cells into two sets of three - each will deliver 4.5v.

I wrapped one trio in plastic tape, and it fits neatly into the battery compartment of my 512ke. It's a bit shorter than the original, so I wrapped a small cube of plastic foam in aluminium foil to pad it out for a firm contact with the terminal at the top of the battery compartment.

Works perfectly, price $1.50 - and I have a spare ready.

Rick