Unless your battery uses a communication signal, you can use any generic charger designed for your nominal voltage (36V, 48V, 72V etc..), chemistry (Lithium Ion, LifeP04, Lead Acid) and max charge rate (2Amp, 4Amp etc...). Then you just have to match the connector and confirm polarity. You can buy them from anywhere like Aliexpress, eBay, Amazon etc... There are also more robust chargers like the Cycle Satiator.