If you can't or don't want to use a Realtor my opinion is that Craigslist and the Greensheet are your best bets. Zillow and Trulia are notoriously inaccurate, so I wouldn't even waste my time with them.
But if you want the best comps, I would use a Realtor (and yes I am one so I'm biased).
Only realtors can access the MLS, @Emmanuel Onyemem , so if you want to know true comps you'll need to work with a realtor or become one yourself.
MLS! I don't know of anywhere else you are going to find those comps. Plus that is what all the appraisers etc, use for their data base.
<----- Contact me, I'm a Realtor/Insurance Agent. @Emmanuel Onyemem