I don't know specifically where to find something like this, but if I were you I would call your local electrical wholesaler - someone like Newey & Eyre. They're bound to know.
Alternatively, have you considered using a PIR security light? This wouldn't need a switch at all.
I've no idea where you get them from, but I reckon that David's right about wholesalers. Our electricians didn't bat an eyelid about fixing one to our larder door. It's really quite a simple bit of equipment - just a press-switch that's compressed when the door's shut, and released to the 'on' position when it's opened.
Do have one. After eight years I still bless it every day.
David's reminded me of the other benefit of these switches.
If the door isn't properly shut (an open invitation to flies and other uninvited guests) it jolly well shows.
(Shame on you, David. If I can learn to shut the door as a matter of routine, anyone can.)
I know, I know. But I've never been able to do routine. I can do something every day for all my life - then suddenly forget it. Last week I forgot to shave one day!
I always tell my wife that it's because my mind is higher things than mere domestic routine - but she isn't fooled.
What a shame! Mrs P. is out at the moment, and won't get the chance to read your post, because she's still got the ironing, cooking, cleaning and dog walking to do.
hmm, there are microswitches that will do that, you'd need to mount it in a fire retardant housing to avoid getting bit, eg any small electrical wiring accessory.
Beware of cobbling something together out of an unsuitable switch type, as the door can close slowly and the switch must always switch very quickly. Not all switches work like that, microswitches are designed for that.
What am I doing? Sometimes I just dont sleep, its the way it is.
The one on my larder door is a bit like the sort of switch you get on table lamps, only bigger, of course. It consists of a plastic box (5cm square x 1.5cm deep) that is screwed to the door-frame; a bit of the wood strip that stops the door swinging out (I know it has a proper name but my mind's gone blank) has been cut away to accommodate it. A spring-loaded rod goes through it, and is the switching mechanism. Door open, the rod on the larder side sticks out and the light comes on: door closed, the rod is pushed back into the box and the light goes out. Couldn't be simpler.