While STAPLES and other stationary stores usually carry a three-fold Presentation Board it is easy to make one in a few minutes.
Typically they measure 36 inches high and 48 inches wide when flat. The basic design has two side panels - each 12 inches wide - and a center panel 24 inches wide. You can create one using heavy cardboard or Styrofoam board. If you can find a sheet of cardboard about 36 x 48 then just score the folds with a knife. Otherwise you can cut the three panels and tape them together.
Sasha dolls don't usually need a background as high as 36 inches so you can cheat a little on the height -- but not too much.
Some stores also sell colored Poster Boards that make good background colors and some even have printed patterns and textures. I have one with puffy white clouds on a blue sky background.
If you want to create fabric background boards you can go to an art supply shop and buy canvas stretcher wood sides by measure and make a background to your own specifications. Art shops often carry pre-stretched canvases that also make excellent lightly textured backgrounds or light bounce boards.