Most insurance agents will determine the replacement cost for you when providing a quote. I would bet that if you contacted another agent for a quote they would give you a replacement cost estimate. If you would like to do one yourself I am aware of a website by Marshall & Swift http://www.accucoverage.com/
where you can estimate the replacement cost for yourself for around $8. We are not affiliated with this company but we do use them with some of our companies to provide replacement cost policies for our insured's.