Go to Google Play on your phone and then to your apps. Swipe over to the All Apps tab to see the list of every app you've ever installed, even ones you've uninstalled. You should find Flash in there and reinstall it. Doing a search for it in Play isn't likely to show it because it's not officially supported on ICS and JB.
There are a couple other ways to try getting it, but this seems to be the easiest for most people.