I'm in the mid-to-late 80s. I put AAA American Association teams in Dayton and Grand Rapids (neither of which will have real-life minor league clubs until around the mid-90s), AA Eastern League teams in Worcester and Poughkeepsie, High A California League teams in Sacramento and Needles, Short-season A Northwest League teams in Ogden and Walla Walla, and Rookie teams in Terre Haute, IN and Bloomington, IL.
If those cities won't work - I know Sacramento and Ogden won't in 2013, since they have real teams now - try Canada. Lots of large cities up there with no teams, or only independent league teams. Or even try Latin American cities like San Juan or Havana or someplace a little bit out there like that. Sometimes I go for "biggest city without a minor league team", others just on a whim or whatever. Or towns that used to have a real-life team, but lost it, like Evansville.
If you're starting in 2013 and working forward, it should be even easier, since you don't have to worry about whether a town will have a minor league team in the future or not. After this year, who knows, unless you're Nostradamus or something.
