MINNEAPOLIS – This is a tough time to be looking for investors in an airline, but these days that’s Sean Menke’s job. Menke is chief executive of Frontier Airlines, which has been reorganizing under bankruptcy protection since April 11. For the past few weeks Menke and Chief Financial Officer Ed Christie have been visiting dozens of potential investors, looking both for an extended bankruptcy loan and a financier to take them out of bankruptcy. They got the first, but still need the second.