Try your local free ads paper under camping and caravanning. Three way(Gas, electric and 12 volt) fridges usually go for about £80-£100. Whatever you do, check on the web for spares availability for the model your going to look at, as models change fairly frequently and you can't always get the burners. If you intend to do a long overland trip, go for a compressor fridge at about £500, as these can be repaired by a normal refrigeration specialist en route if they go wrong.
If you want to save money and just want to keep milk and butter cold at a fixed camp site, try the following. Put whatever you want to keep cold in an open bowl full of water and cover it with a soaking wet towel etc. This works by heat loss as the water evaporates and is a really effective way of keeping stuff cool without going to the cost of buying a fridge.
The other drawback of fridges is that they do take up a lot of room, even in a 110. I hope this helps. Cheers, John