I don't know where they got the dog tags they used for the show, but I got mine here: usamilitarymedals.com
To get as close to the show and actual military, I picked 'Dull Finish Dog Tags', 'Stainless Steel Dog Tag Chain' and 'Black Rubber Silencers Milspec'.
You get two tags, one large chain and one small chain, and two silencers with your order. Not bad!
You also have the option to customize your dog tags. Here is a picture of 'Samantha Carter's' prop tags from the show:
You can use the numbers on these as your 'ID' or you can make up your own. Don't put your real birth date or your soc. sec...Just don't. Use part of the serial code on a gatorade bottle or something.
And once you have them, don't lose them! Or let them get sucked into a black hole!
Here are mine!
If you take good care of them, they'll last for a reeeaaally long time!
Proof!
{WarriorClassStudios}