You might can go here http://www.wesgometals.com/ . They've got it listed under the alloys. I don't know if they actually sell it or what the price would be. Perhaps you could post that if you find out.
silver cold plate? what dimmension do you need if it is small you can get silver for cheap at jewelry stores or jewelry supply stores and cut it yourself. should cost about 10-15bucks for a 4cm square!