Unless Apple are still signing the ios5 blobs, it's unlikely you can. Easiest way to find out though is give it a go.
Has a link to each of the ios5 firmwares for various devices. The downloads are actually hosted by apple, I wouldn't recommended obtaining firmware from any other source.
To install, you'll probably have to google it, but if memory serves, load itunes, select your device, hold down the option key and select restore. You can then pick the firmware. It'll only be installable if apple are still signing that firmware though.