I think in the long run, as craigepo said, there's no substitute for a quality run outsidebut here are some alternatives I have used with success:
1) Running in an indoor track
2) Running stairs in an office building
3) Pool Running
With a little searching in your AO you might be able to find somewhere that you can do some sort of running-like activity. In highschool, during indoor track season we ran around a 100m square (school hallways) on cold days. I've even been forced to do suicides on a basketball court as an alternative. Just try to think outside the box