Is this your bike?
Unfortunately, it uses something like the common 36V Silverfish battery but not quite the same. Doesn't use a rear strut and the key is on the wrong side. so it must attach quite differently. I doubt that you're going to find a replacement,
You don't have the old battery?
A good ebike hobbyist could find a way to fit a new silverfish or maybe a Hailong style battery. It's a labor of love though to do it right, with no rattles or disasters.
Get another bike?