just bought one as it happens - united british caravans on sandy lane, £29.99
we're going 'nomad' for a few weeks island hopping in the boat in scotland so that what its for, can't be doing with warm beer beer on an uninhabited island, oh yeah and apparently wor lass has requested some room in it for her insulin, these women, its just self self self
you can get them that run on their own power source, think they are known as cars, you just plug it in to one or something
you can also run them from something called a 'shed' which is what I tried as soon as I got it home, seems to work ok. If you just plug it in it takes a few hours to get really cold, but according to the manual, if you give it a head start by shoving some ice packs in its lots more efficient
best bet, if you want totally self contained is a 3 way one that will run on gas as well, you can get them that you put camping gas canisters in but they ain't even a little bit cheap the one we looked at was £165