You will need a 9.6V Battery with a "Tamiya" style plug, this is the standard size in most toy cars back then. You can find them here or other large sites for $10-20.

I have tested the buggy and as of listing it is fully functioning. I've cleaned it up a bit but can clean up better. The transmitter on and off switch is a little finicky, you might need to kind of wiggle it to get the red light on. Probably an easy job to take a part and just clean the contacts. Just beware.

Because of the age of the toy, it's as-is and no guarantees or returns.