You could try ripping apart an older caluclator or adding machine. I tore one apart once since it had stamped metal triangular shaped "domes" under the buttons, the 3 corners acted as one contact and there was a dimple in the center that was the other contact. Pushing in the center would make it sort of "pop" down in the middle and make contact.
Opps, just read vacpresses last post. Same thing.
Also chech out these very thin switches from ALPS
http://www.mouser.com/catalog/625/1167.pdf
I think they have a similar metal peice in them that you could maybe rip out and use in you own design.