I get mine from:
http://www.loveyourpets.com/
$3.95. At that price I got enough to put them in the laces of all my shoes, on the zipper of my jacket, and one around my neck. And got some for my pets collars too as long as I was paying shipping.
I agree that it's a great idea, but there's nothing magical about RoadID. I bought one once but it's just a damn tag, there's no reason to pay $20+ for a tag with some stuff etched into it. Of course, if you have medical issues that are relevant in emergencies and can't be adequately expressed on a MedicAlert tag, you might want to go for their extra services, but for most people a literal dog tag is fine.