Flash is no longer listed under Play Store, however is still possible to download and install it from Archived Flash Player Versions page on the Adobe website.

1. Download the Flash Player apk from the archive page:
Archived Flash Player versions
2. Install the apk
3. Restart your browser
4. Done

Quick links:
Flash Player 11.1 for Android 4.0 (11.1.115.69)
http://download.macromedia.com/pub/f…player_ics.apk

Flash Player 11.1 for Android 2.x and 3.x (11.1.111.64)
http://download.macromedia.com/pub/f…er_pre_ics.apk