I do not know much about that area of law but it stands to reason that the states having more population growth and business expansion are those where the government is more likely to exercise eminent domain rights to build more roads, transit systems and other infrastructure construction projects. In addition, eminent domain comes into play in those states having natural resources such as oil, gas and mining operations. I located an interesting website http://castlecoalition.org/get-information which appears to be a coalition of activists dedicated to fighting eminent domain abuse. There were a number of interesting books offered for sale on their website. You might want to look into some of these books as they some of them would give your more information about where eminent domain activities are more prevalent.
This is not my area of expertise, but I would encourage you to look at future problems that may require eminent domain solutions. I would think that addressing global warming related sea rise will involve the building of significant coastal infrastructure. Regions with rapid population growth will also encounter more eminent domain issues. With that said, I echo K's advice about related fields - look into land-use and general property law as they are often practiced together and are more ubiquitous.
I am not an expertise in this area, however I would advice you look up to states like NY and Lousiana, and at the same time do your own research and study other areas of law too to be competitive in this field.
I am International Lawyer, hope we can work together once,
For the most consistent work in building a career in eminent domain, you might want to consider doing it at the federal level. The U.S. Court of Federal Claims (where I clerked) has jurisdiction over all 5th Amendment takings claims. Though sometimes the judges there do travel for cases on occasion, the most consistent work would be in Washington D.C. But eminent domain itself is not that common of an issue, so you might want to consider learning about related fields like government contracting.