Not sure if you are asking for suppliers/manufacturers or retail outlets where you can buy them but most chandlers will source fridges for you. They usually have a very thick catalogue full of every imaginable thing you may need to replace on your boat..including fridges, loos etc which they will give to you if you ask.
not comet, cos they're 240v. You need 12v. So, since it'll be a bit weedy, make sure it's one that opens at the top so the cold air doesn't fall out. Also find someone who sells ice to give it a starting chance.
If you want pukka built in then chandlers. Eg www.cruisermart.co.uk Will be £500 up. Maybe caravan shops too?
If you want diy just buy the refridge gubbins and make yer own insulated box. Still a few hundred quid
Maybe one of those motorised cool boxes from Argos/Halfords would do. Like a normal cool box but there's a tiny fridge unit in there and you plug into the cigarette lighter. Only £100 ish. Will not make ice of course, just keeps stuff a bit chilled
Matt, not sure I buy the "open door and all the cold drops out" theory. Yes it's true but the effect is tiny. The cold air drops out and gets replaced by ambient air but the heat capacity involved (ie the energy you need to take out of say 3 cubic feet of air to drop it from 20c to 0c) is tiny. Much much less than the energy you have to suck out of 3 cubic feet of beer or meat to lower its temperature by 20deg.
Or ... as almost someone suggested - buy a high efficiency fridge system and fit it to your existing fridge instead of the gas which [I believe] should NEVER be used on a boat?
Re cold air, etc - you can help the fridge in a few ways! I always try to remember to grab the cool blocks out of the freezer and put them in our coolbags and when on baord stick them in the icebox of the fridge! I have also seen suggested that you can 'fill' any unused space with something that will reduce the air volume in the fridge to a minimum eg. polystyrene, insulated boxes [tupperware type], even poly bags with polystyrene chips would be good - anything that will displace the air!