For anyone who hasn't seen one, they are specially shaped pillows with cutouts for your mask. Most come with a matching pillow cover for washing. Your mask hangs off the edge of the pillow while your head remains supported. They can be stuffed like a traditional pillow or made out of memory foam. I personally found the memory foam version too hard and it still tended to knock my mask off. I've had my filled pillow for years now, and it was a game-changer for me. If you Google "sleep apnea pillows" in Google images, you can see all sorts of available designs.
This is a picture of the pillow that looks closest to what I use:
Mine was about $50. I've seen versions that are much cheaper - like $15 - but I worried that they weren't as well made or supportive.
I hope this helps some of you! All the best!