Check our blog What Are Traditional Mattress / Futon Mattress Sizes?
Removable futon mattress cover
Huan Japanese Floor Futon Mattress, Folding Mat Thicken Matt, Soft Bedroom Student Sleeping mattress cover shikibuton-Gray Full (Size : 90×200cm) : Amazon.co.uk: Home & Kitchen
HBFG Japanese Folding Futon Mattress, Thicken Floor Mattress Twin Full Queen King Tatami, Soft Mattress Pad Topper for Guest Room Sleeping Pad : Amazon.co.uk: Home & Kitchen
Natural Cotton Japanese Futon Mattress | Japan Objects Store
Japanese Futon with 100% Cotton Batting - Arpel Bespoke Home Furniture
Japanese Futon Mattress Topper Futon Topper Thinner - Etsy UK