BiggerPockets

The MLS is often best because it includes additional data like days on market and any seller concessions.

Public records, in some areas, contain sales prices. Some counties around Denver have search functions that let you search for comps in a neighborhood. Some counties have systems that are so obtuse you will pull your hair out getting data.

Title companies can sometimes provide data, too. If you've worked with a title company, give them a call. If not, start calling around and ask if they have any sort of online system for looking up sales.

Zillow can actually be useful in this regard. Put in the subject address and look for comps. Take the "zestimate" with a grain of salt, but the comps are real.

In all cases, you have to do some examination of the comps to see their condition and actual configuration. Ideally, drive by.

How about clicking on the "Real Estate Tools" menu at the right of this page, and then selecting the link called "Find House Values & Comps". Or just use this link that is where you end up by doing the above:
http://www.biggerpockets.com/house-value-comps.html

Theirs always http://www.dataquick.com/ but they charge a monthly fee

Using any of the above should give you enough info to structure your offer - however, once you have it under contract pay the $85 to get a valid desktop appraisal or BPO.

It has saved us a couple of times during our "inspection period" and we successfully adjusted our offer using the new data.

TTFN,
Greg