I don't think anyone makes anything like that. But you could yourself. Get an automotive AGM battery (I've seen them used for about $50).
A regular car battery would work too, but a 'deep cycle' battery would be better. AGM is a deep-cycle battery (meaning it's meant for running electrical appliances, not for starting a car, though it will do that). It can be operated in any position and it's maintenance free. It won't leak even if you were to bust it open with a sledge hammer. So you can mount it in the trunk of a car and not worry about it.
For some laptops you can find a 12v battery charger meant to be plugged into an accessory socket in a car. Otherwise you could use a regular 110v battery charger, like you'd use for a car battery.
Alternatively, if you want to get fancy, you could build your own battery bank of 18650 batteries--the same kind of cells that are in your laptop's own battery. Banks of 18650s are used in everything from portable power supplies to laptops to electric bicycles to electric cars! But it will take some research and also some 'engineering' skill to calculate how many watt-hours you need, and so forth.