1. A realtor will be happy to give you the info from the local MLS if you are going to need a realtor to represent you as an agent. The MLS tracks only sales by realtors.
2. If you can go to the local courthouse they can help you gather this information. This is free and if you are going to be investing in the area this is perhaps a valuable experience. The staff at the courthouse is there to help you and in my experience they are usually most informative, know valuable information about specifics in the marketplace and are good resources to have had a face to face with when you need inormation from them by phone.
3. The local title company can be useful. Ask if they will compile a list of sales for you. They will charge you.
4. If you are interested in duplexes LOOPNET.COM tracks sales by town. You can get information for free or detail on sales by paying a fee.
Best of luck.