7inch Twin Mattress and Pillow Set

Overstock

Rectangular mattress form factor, Light blue color is similar, Subtle quilted/textured surface is consistent with frame's appearance, Could be a twin size

NapQueen Rainbow 5" Twin Memory Foam Mattress

Home Depot

Rectangular shape, Blue color, Mattress category

Navy Blue Twin Foam Mattress

English Elm

Rectangular form factor, Dark blue color is a good visual match for the frame's mattress, Quilted texture is plausible under the plastic wrap