Try DFS in San Javier - if they still exist.
There is also another furniture shop that sells mattresses. If you go into San Javier from the rear entrance, turn left at 1st roundabout and it is on the right hand side before the next roundabout.
There is also a furniture shop on the industrial estate - go to Roda viallage, turn left at the lights and it is in the industrial area on the outskirts of Los Alcazares.