Batteries for Yamaha TG500 tone generator memory card

[merovie]

I've seen coin/button cell batteries for sale online for the MCD64 memory cards but they don't specify what size or number the card uses, probably so they can sell them for 10 bucks when I am pretty sure I can find the right size one at WalMart for 2 bucks.

Does anyone know what model coin cell battery the TG500 tone generator memory card uses? Yes, I know I should have thought of that before I removed the old one and lost it, but nobody's perfect