You really can't most places. If you're dealing with a shopkeeper who owns the shop and pay him enough extra to be worth the trouble of exchanging, maybe (offering $10 for a purchase worth $5 isn't going to cut it though, he's not going through the hassle for $5). If you're dealing with a cashier who has no decision making power, like you are 99% of the time, then the cashier simply cannot accept your foreign money. Nor can a bus driver. Nor can a vending machine.