Asking your Realtor is a good resource. The comps will be accurate and current. I have a new program that helps me to be even more detailed in accumulating comparables. You can use online sites like Zillow, but often their accuracy is flawed. Usually a Realtor will provide Free Comps for you.
Bonnie Frazier, CDPE, GRI
Your agent should provide these to you. If you do not have an agent and want to sell your home, you should get one. If you are a buyer, you also need an agent to help you find the right home at the right price.
If you want comps on a home, just send me the address and I will send them to you. It takes me about 5-10 minutes to pull them for you.
You can also Google comparable sales of homes and find some sites that provide fairly accurate results.