Martin, was looking at your old post on .410 flares. I don't know of any source but Orion has a large selection of various flares including a good looking 12 gauge flare. Take a look:

http://www.orionsignals.com/Marine/Products/aerial.html

They make some good hand held flares. I bought a bunch at WalMart when they discounted them. They would definitely get someone's attention. As I remember, they were about four or five inches long and about the diameter of a 12 gauge shell.