If you have the original grey install DVD that shipped with the iMac you should be able to insert the disc and restart while holding the C key

Replacement disc(s) from Apple:  You can get replacement System Install & Restore CD/DVDs from Apple's Customer Support - in the US, (800) 676-2775 - for a nominal S&H fee. You'll need to have the model and/or serial number of your Mac available.

Downgrade OS X Lion To Snow Leopard [Video How-To]

http://www.cultofmac.com/110614/downgrade-os-x-lion-to-snow-leopard-video-how-to /

How To Downgrade OS X Lion To Snow Leopard: The Complete Tutorial

http://www.redmondpie.com/how-to-downgrade-os-x-lion-to-snow-leopard-the-complet e-tutorial/

How To Downgrade from Apple OS X Lion to Snow Leopard

http://www.pcmag.com/article2/0,2817,2389334,00.asp

Lion wasn't released until July of 2011

Insert the original  gray install OS disk into the DVD drive. Press the oprion key the Sytartup Manger will open.

Select the DVD

(see http://support.apple.com/kb/HT1310)

Are you using the ORIGINAL DVD's that shipped with your Mac? If you're trying to boot from a version earlier than the one that shipped with your Mac or DVD's from another mAc it won't work. The version will be listed on the grey DVD

Get a caddy and second hard drive and then you load snow leopard on the second hard drive and then when you need snow leopard start your computer holding down the alt key as you start the computer and then the computer ask you for the hard drive you want to run from so then you have the best of both. I do this with my emails accounts so i can bounce emails in snow loepard to make it look like the account is closed or its not there anymore, but apple took this out of lion for some reson, this was told me by apple staff at london store to get the best of all snow leopard and lion and I will do this with mountain Lion as well. as with snow leopard I have Windows XP on the boot camp so I still can use programs that work with XP and then with lion I have windoews 7 running the same with boot camp.

You can purchase snow leopard disks from many many places, your just not looking hard enough.

You can actually install snow leopard onto a machine that came lion, but if your not a very advanced user than it's pointless trying to explain.

Give up and upgrade to Mountain Lion, happy days.

Boot to Snow Leopard Install DVD -> Select English -> open Disk Utility -> Select the Hard Drive (not macintosh hd) -> click the erase tab, selecting Mac OS Extended (Journaled) -> name the hard drive what you like -> click the erase button -> once finished, go back and select install Mac OSX Snow Leopard -> Done.