I've been looking everywhere but I cant find it
ebay, but they're not the best ones. if you can't find any on the internet then you can check some jewelry shops or the market to see if they sell some chains (might not be specifically for dog tags but you can still use most of them)
you should be able to find dog tag chains at BRIGADE QUARTERMASTER
WEB ADDRESS- BRIGADEQM.COM
they deal in military items, anything you need inclusing dogtags- you will find there
claires has a lot of dog tags. they are really cool.