Solid (NOT the shredded) latex pillow of any brand. My partner had one from the time she was a child to 30 years later. She convinced me to try them out and now I'm never going back. If you're someone who needs a lot of support (like me), having a firm pillow to put under the latex one is a good idea, but it should be perfect for a side or stomach sleeper.

They generally run ~50$(us) for standard-size and can get over 100 for a king-size, but it's definitely worth it in my opinion.