1A or 1000mA generic usb charger works fine for my LG G3 while in use. 700mA works too, but while in use the LG G3 uses power faster than 700mA can charge it.

You'll need a micro USB cable too.

http://www.differencebtw.com/wp-content/uploads/2015/02/Micro-USB-and-Mini-USB.png

Reading around, 1.8A or 2A works too.

This one looks solid for $6 at the moment, good reviews-
http://www.amazon.com/Charger-UBP-Universal-Smartphones-Tablets/dp/B019C2B13G

This one has a 1A and a 2.1A together-
http://www.amazon.com/KingAcc-PowerSmartTM-Technology-Motorola-Devices-1/dp/B00U4UAEV6

I wouldn't recommend getting one of those tiny chargers unless you need to carry it around a lot.