Some of the more common resources for comps on specific homes are the local tax records, trulia, and zillow.
Without a doubt, the best route to go is to be in touch with a real estate professional that will help you apply this information and improve your chances of success.
I go to the tax records to see what subdivision or community the property is in.
I go to "CMA," which is pretty much the basic search function. It defaults to transactions within the past 180 days. Generally, for starters, I'll leave it at that.
I then search for properties within the community that have sold (or are still active) within the past 180 days.
I then narrow down the search as appropriate. Usually, in a particular community most of the homes were built within a few years of each other. If not, I narrow that down.
I also narrow it down by number of bedrooms and baths, but leave some flexibility there. For example, maybe the homes were built as 4/2s, but some owners have added a third bath.
If there are some other unusual or unique aspects, I'll include those in the filters as well.
My goal is to end up with 3-5 very similar properties. From there, I look at the listings and see whether the properties differ in some way from the subject property. I make adjustments accordingly. I also look at the seller subsidies, to see what those were.
In other cases--where there isn't a community--I'll do a geographic search, generally (in suburban areas) starting with a radius of about 1 mile. I then narrow it down the same way.
In an urban area, or if I'm looking for comps for a condo, I'll start my search just on the condo itself. Or within a block or two of the property.
And that's how I do it.