The best place for preforeclosures is your county clerk's website, or office. Preforeclosures and foreclosures must be filed with the county.
RealtyTrac.com is a website that lists these properties. They do have a free trial period, but it will cost you eventually.
Your best bet, as already stated ,is to find a good agent willing to do the research for you. Please remember, real estate agents get paid only when the deal closes. So if you decide to hire an agent, follow through with them. You won't be paying them, the mortgagee (lender) will.