Market cap rate is primarily a function of market demand. Not the answer I think you were hoping for, but I believe that to be the reality.
More investors in the market = more demand = higher prices (and thus, lower cap rates)
Think of a cap rate like a bell curve. Every market will have investors who are willing to pay less than the market cap and investors who will only buy if it's higher than market, but the average investor in that market is going to pay that cap rate.
Large national brokerages track sales and cap rates from those sales and can be a good source for generally reliable market rates, but the reality is that there is no one source with the exact (agreed upon) cap rate.
It's very similar to residential in that it's not an exact science. Residential relies on comps, which are also subjective. That's the "beauty" of real estate. Lots of companies spend lots of money trying to determine market rates, but it will never be perfect.