is there a website where I can check on rental rates for our area?

The best site in our area is Craig's List. If they don't have listings in your area, check the classifieds of your local newspaper.

There will not be a good web site to accomplish this. The only thing you can do is physically find other properties for rent and compare. If you ask other similar business how much they pay in rent that might help but they may have had a lease for a long time and may have a low rate.

But every rental web site I have ever seen is way out of date and very incomplete.

Are you talking about a commercial business? That is a different kind of market than residential real estate. Your local chamber of commerce might have a ballpark estimate on what is going on in general, but for specifics talk to a local commerical real estate agent. You can find out what some average costs are per sq. ft. That is what you usually see commerical space listed as, cost per sq. ft.