Which websites can I go to find home sales prices for property value essment purposes?

I want to reduce my property tax on my house and I recently had an appraiser do an appraisal on my property. The value was over $100K less than what I paid for it, but I found out that I need homes that sold around March 31, 2008 in my neighborhood as proof, not this most recent appraisal. Where can I get such info? Thanks!!