You can get a compact fluorescent bulb (CFL) in pretty much whatever wattage you want at any hardware store, Walmart, heck most grocery stores even. Are you growing plants? A 9 watt (actual wattage, probably a 40 watt replacement) would probably be fine, and cheap!
ETA: Like this. You can probably find them individually too.
I would think a 13 watt would be good. But make sure it's daylight spectrum (6500 k), and not soft white or bright white- it should say on the box. I would say your best bet would be Home Depot or Lowes. I got a 4 pack of 13 watt daylight bulbs there for like $8. And you have a better chance of having someone be able to help you find what you need, too.