Probably the easiest and cheapest option is to send it to Jimmy at
http://ebikebatteries.co.uk/ for a re-cell. He'll make it better than it was when new.
Alternatively, you can use any rack battery. You can buy them with the rack or just a fixing plate to fit on your existing rack. You'll need to adapt the fixings slightly and take the controller out of the little box at the front and put it in the new one. Any 36v rack battery that you can find on Aliexpress should work.
To give you some idea of what I mean, here's a Schwinn Tailwind that I did.
before:
After: