I HATE to mention, the 'C' word (no NOT the rude one!) but I have just bought a Nice set of Christmas/Fairy Rope Lights. I paid under £10, including delivery. (Please don't 'tell me off', too much, for this people....I'm sorry.) They are power able by either three 'AA' batteries, or a USB port. USB Ports apparently give 5 Volts at 500mA- I looked that up.
Sorry about the 'C' word again.