iLife System Requirements:
(Covers all versions of iLife up to iLife 9)
iLife 11 is only available from the Mac App Store and therefor requires Lion or above.
You can get the boxed retail version of iLife 11, and it will run on Snow Leopard. You will be able to run Software Update, but you can only update iMovie to 9.0.4. With Lion or Mountain Lion you could go to 9.0.8.
The retail boxed version is no longer sold by Apple, but you may find it on Amazon or eBay.
You also might try contacting Apple Mac App Store support and requesting that you be able to puchase 9.0.4 from the App Store. I don't know for sure that they will sell back level versions, but anecdotally, I have heard that they have for others.