A leisure battery is the cleanest/quietest/simplest way, available from any camping or caravanning shop.
Or you can use a battery designed for electric scooters/wheelchairs:
http://www.rapidonline.com/Electrica...atteries-73327
The capacity needed will depend on what you're doing. A full weekend of touring car racing with tyre warmers etc will need something like 80Ah. For a day of off-roading pretty much anything will do (I can charge all day off a tiny 20Ah gel battery designed for wheelchairs).
You can charge off your car, but car batteries aren't really designed to be deep discharged regularly. That said, charging Lipos doesn't take that much out of it, so if the battery is in decent condition you should be fine.