Eco-friendly bulbs can be purchased at a variety of hardware and big box stores for under a buck a piece. I've even found them at discount stores like the 99¢ store. Online, they tend to be a bit more expensive, however, if you are looking for a specific type of CFL bulb, like an indoor floodlight, online stores and websites are a great resource.
Did you know Home Depot has a CFL recycling program? Find out more details here