When I was in Japan I bought this miniature backpack, about the size of a wallet. The sporting good store was in Ochanomizu (御茶ノ水). It lasted me about 5 years. Now I am back in the United States and I have trouble finding a new one.
I can tell the brand is EIGER, but I can't tell what this model is called or if it has been discontinued. Normal sized backpacks are easy to find, but not this miniature size which fits in your wallet.
As you can see, I stuck a chain and a carob-beaner to attach it to my pants. That's just using stuff I found at the house and hardware store.