Go to a nearby cellphone store!
When I run out of battery on my cellphones and if I am not carrying along a charger, I often go to a nearby cellphone store and ask them if I can charge my phone. The cellphone store you go to, however, depends on your mobile phone operator (the biggest ones being Docomo, AU and SoftBank). You should go to you respective phone operator if you want to charge your phone. This is an easy and straightforward way to get your phone charged!