I don't get how finding a store that stocks the batteries, going there, buying it, charging it, packing it, dropping it off, is any easier than simply finding someone with a charger and borrowing it for a few hours.
Modern passenger planes have USB charging ports at the seats. Even some airports are starting to have USB charging ports at the gates. Some modern hotels have USB ports in the rooms. Or I've seen some hotels that have a stock of abandoned USB chargers that past guests have left behind, and they will freely lend them to any guests who forgot their own charger. Or just go to a local convenience store and they usually stock some chargers.
Of course, I am sometimes worried about plugging an unknown USB source into my phone, about short circuits or viruses, but I think the risk of damage is low. With the upcoming debate on the EU directive for common smartphone chargers for the purpose of reducing E-waste, I think the problem of trying to find a compatible charger when you've forgotten yours will be resolved.