Any local Realtor can help with this issue. In fact, you are right in saying our MLS system is the only workable data set that can be used. (You can go to the county records but you can't sort and search in a computerized way.)
I suggest that you call your local Realtor, or, if you don't have such a relationship, try to build one with an active agent. Most of us can do the work you need quickly and are willing to do so in order to build a referral network. We help individuals and hope that they will help us by recommending our competency to friends and family. We are, in fact, as you say, the "local business community" with the information and weâ€™re not hard to deal with, honest.
By the way, I believe that you have to be very specific about tax comps. You have to show that, in relation to other properties in your local area, your property has been overvalued.
The over-all township rates are not useful. After all, over-all, the total property asset value in the township is the basis for the tax spread. Some people think that, if the value of their property goes down, their taxes will be less. Far from it. If the decline is more or less universal across the township, taxes on any individual property will remain the same. The Town's expense budget is the same and the property owners have to contribute the same amount to balance it. They do so regardless of general shifts in their net wealth, whether it is that of their Real Property or the value of their investment portfolio.
You can look me up if you like.