There isn't one. Not real comps.
You can develop some, presumably, through online tax assessment records. That's public information. And then you could make adjustments. It might be best, though, just to pay for an appraisal. Either that, or ask a Realtor who's experienced in commercial property to do a CMA for you.
As for the deadline within which to appeal, that'll depend on your jurisdiction. Check with your local tax assessor.