Both Trulia and Zillow are good sources to find out what the current home owner paid for the property, which they get from the public records. Often local periodicals will also track that information and you can look it up in their archives. If still not finding the information your looking for, you can always contact your local realtor and they should be able to get that information for you.
That varies from area to area. Here, in the Florida Keys you can access the property appraiser's website and find it. You can also find it through tax records. Many tax authorities provide it in user-friendly websites, others make it much harder to find. But it IS a public record and you can get it, even if you have to actually physically go to the public office where the records are held.