Sofa bed is a folding mattress 120cm width. Here you will find for this kind of mattress replacement covers and bags.
Replacement cover for mattress 200x120x10 cm
60x120x50 cm folded
200x120x10 cm unfolded
Natalia Spzoo folding mattress 200 x 120 x 10 cm
Tested for harmful substances according to Oeko-Tex Standard 100