That depends what you want to run off it, if it is just for charging phones/lamps etc then a smaller one (under 1KWH) but if you want to run a freezer you might want a bigger one. Plenty of good ones on Amazon.
We have a few, an old small but easily portable Allpower, a 1000WH Vitoman and a similar sized Jackery one.
https://www.amazon.co.uk/s?k=camping+battery
Edit: we also have a 2KW petrol generator and solar panels for the camping batteries.