just a small local autoparts store..
Bulb wattage is no secret, although most people don't care, and just buy them by the replacement number/type, like #1156 ..
The 1156 is single element, 21 watts.
The 1157 is dual element.. a 21W(stop lamp) and a 5W (tail lamp)
ask the guy at the parts counter .. or try different stores.
"here's a page with some common dual-element bulbs and wattages .. also candle-power ratings":http://www.webbikeworld.com/Motorcycle-technical-articles/triumph/brake-lights/light-bulbs.htm
"...data is from one manufacturer's bulb specification guide..."
Notice they are all close to the same wattage .. standard auto bulbs..
Although all this little stuff adds up in terms of maximum load on the magneto, there's no need to get too precise or technical with this .. just find a 12V bulb that fits the socket and is bright enough for government work..
A head lamp might be a different story. They use lots of electricity.