Adobe has a beta version of Flash that's supposed to work on Leopard. You can get Flash Player 9 Update beta from:
(Hat tip: http://www.colettas.org/?p=197)
But the Adobe installer is a mess. I had a hard time getting it to work. I had to google the specific error message about not being able to install a file. I found the answer which involved removing some old installed files, repairing permissions, rebooting and trying again. I can't remember the exact details, but it took a lot of trial and error before it worked.