My guess is that you are out of luck. The latest requirements for iMovie '11, as listed on the App Store, are "OS X 10.7.4 or later". In other words, iMovie '11 (in it's latest iteration) will work only on Lion or Mountain Lion (or later).
My wife's iMac is on Snow Leopard OS X 10.6.8 and is running iMovie '11 version 9.0.4. Checking for updates returns the message that iMovie is up to date. So it appears that the last version to run on Snow Leopard is 9.0.4 - hence why you can't download 9.0.8 to Snow Leopard from the App Store. Perhaps you can purchase a boxed version of iLife '11 somewhere (possibly eBay) that has an earlier version of iMovie '11.