I get tags from PetEdge.com - You can buy them in packs of 2 so it's perfect for my 4, very cheap too.

I have so many collars and leashes that I don't have one designated place to buy them. Just wherever I've found one I liked. I've bought them before from: PetEdge, Petsmart, Petco, Target, WalMart, KMart, Local dog places, DogLovers, Petsupermarket, several others. I could probably open up my own shop I have so many.