"Are there resources to finding those unicorn consultancies?" Great question. AFAIK, there are not. I have tended to only find out about them by word of mouth

"Are there resources to finding those unicorn consultancies?" Great question. AFAIK, there are not. I have tended to only find out about them by word of mouth after spending some time in a particular area of specialization. Back when I was doing Flex, Universal Mind and Digital Primates were well-known in that arena. You tended to see them in conferences, and some of their employees were the ones who authored the best known technical books on Flex. So there is one idea: find out where some of the renowned bloggers, authors, and developers in your field of interest do their consulting.