I don't think they are universal, unfortunately. I'm pretty sure each brand has their own stuff.
They can be used on any crescent shaped pillow. Just not the new version they have come up with. I got my " boppy" cheap at Joanne fabrics. It's not boppy brand, but I use the covers.
Oh I didn't think to look at a Joannes fabrics for a Boppy/pillow. I'll check it out. Thanks!