Here's something I posted the other day that might be helpful...
I decided to start up a fictional league and use a mixture of current MLB cities and new cities. Of course, the question then became where to find good population data. My first thought was the Census Bureau, and I found the page <a href="http://www.census.gov/population/www/cen2000/phc-t3.html" target="_blank">accessible through this link</a> quite helpful. Its the Ranking Tables for Metropolitan Areas: Population in 2000 and Population Change from 1990 to 2000.