Leisure batteries are still common enough if your racing outdoors...much quieter than generators trackside, and no petrol to handle either
...but only give you 12v...rather than a 240v mains supply that a genny will give you, which for me is perfectly good.
Most chargers run off 12vdc so need a 12v supply to run, so leisure battery or dc power supply as has been suggested already.
However, there are also chargers that have both a 240v supply and 12v built in...I use the etronix powerpal which has that feature, and charges all the current battery types.