BiggerPockets

Try homegain.com; realtor.com(free) and icocw.com(pay).

Joe, I use an actual appraiser to get the real comps. My agreement is if they give me the comp then they will get the appraisal on he home when I either sell it or keep it for myself.

This link should provide you with some useful information:
Find House Values & Comps

Harrison is ABSOLUTELY correct. I shared the link above to get you to the sites you were looking for, but as I say over and over again, NEVER rely on the information from these sites to make any business decisions!

Particularly in the current market, the only data that matters are recent sales.

Jon -
Logically, you should look at current listings too, but I've seen too many that are simply priced WAY above any reasonably realistic value to find them of any value. I guess that's not going to be true for all properties, so I'll relent and say that current values should be considered, but be wary of the outliers (something you should be doing anyway)

The back end of the MLS will tells you some very important information about what's going on in a particular market. What Zillow, Trulia, and the front end of Realtor.com won't tell you is if there were any concessions, how many days on market, and the overall jounrey of a property. This information can be very helpful in trying to calculate what a property is really worth.

Say an out-of-town investor is looking in a new area to invest and there are two properties at the same price. Let's make it more specific. Say a Los Angeles investor is looking into the Inland Empire (ground zero for foreclosures in Southern California). Being new to the area and used to LA prices they get excited about the prospect of buying a home at $135k! However, little do they know this particular area is on the outskirts of active comps. One home is listed by a retail seller unable to lower their price any further and the other property has been listen by an unrealistic bank for over a year. If this investor had access to the MLS, they could have seen the REO property's journey through different agents and the tremendous price drop in one year. They also would have realized both were not market value since there were no good comps in the past several months in the area AND the REO still hadn't sold after being listed for over one year dispite a drop by 50% in price. This is actually a very real scenario in San Bernardino and Riverside Counties right now.

Build a relationship with a Realtor who understands what you're trying to accomplish. Find one that's been through a down cycle before or one that's ready to write offers for you and get hit over the head a few times.

Build their commissions into the offer, let them represent you at auctions (doesn't cost you anything), and/or let them represent you when you're selling the proeprty if you're flipping. I think a good Realtor is one of the most important pieces to your team. As an investor, it's hard to know and do everything. A good team member might just save you a world of hurt on the other side.

Also, I am a very visual person and do pop my head into RealtyTrac now and then. I like to see what's coming down the pipeline as far as REOs go. Everything is a little screwy right now because of the hiatus of some banks on their REOs but when it's working, you can see the homes in the NOD process, trustee sales, and those already REO. Match that with what's listed on Realtor.com and you get a nice sense about what's going on in the neighborhood and what you can expect after holding the property 90 days before you sell.

You should get out and learn your target area. I assume you selected the target area based on some criteria, so you should be able to learn that area or areas.

From what I hear, I wouldn't expect many phone calls until you've done a few rounds of mailings.