I also am a side sleeper and use multiple pillows when on a bed.
Backpacking I've tried:
- Fleece-lined stuffsack - worked well when I took too many clothes but not anymore
- Flex-Air - Bought a case of the dual-chamber version but didn't love them and sold most of them in Gear Swap
- Thermarest Lite-Seat - This worked okay when I let some of the air out and folded it in half and put rubber bands on the ends, but again it wasn't great
I've got my eye on the Cocoon Hyperlite Pillow - I actually stopped at REI to pick one up but they were out of stock so I don't have it yet.
I've switched to mostly hammocks while backpacking now and find that I really don't need a pillow at all. While I'm a side sleeper in a bed I find that I'm very comfortable on my back in a hammock without a pillow of any kind.