Amazon has 23" x 23" Best Friends by Sheri The Original Calming Donut Cat and Dog Bed (Lux Fur Oyster) for $18.51. Shipping is free w/ Prime or on orders $25+.
I have one of these for my cats and one of my cats is obsessed with it and will sleep in it all day long. Gonna grab another one so my other cat has one too.