SONU brand mattress made for side sleepers. They have a giant channel where the pillows go so you can slip your elbow under your head without cranking your shoulder out. Super sweet bed but the one I got was too firm and now it's past the return window for the mattress. Comes with two sets of sheets designed specifically for the bed, and both the softer channel style mattress fillers and the harder style so you can turn the mattress into a normal mattress if you want. Here's a link to the sonu site for more detail on the mattresses
https://sonusleep.com/products/sonu-sleep-system-firm?variant=40035683467382
The one I've got is a couple years old now but barely slept in and stored in plastic, just need to get it out of my storage unit. The mattress alone is usually $2500- $3000,
$400 OBO takes it plus all the extras for it.