Check out the insulation area of Home Depot or Menard's. They sell sheets of foam board insulation in 4-ft x 8-ft pieces. Depending on thickness, each piece is between $8 and $14.
My dh says they can be taped together with masking tape, but I haven't actually tried that yet. You can use duct tape to tape two pieces together on one side and then leave them there as a hinge (two pieces folded together for storage with the duct tape hinges on the inside).
I got my boards to construct a quilting table as demonstrated on Youtube. They can also be used to set up a design wall.
The biggest problem is getting them home if you don't have a truck or van. We have only sedans, so dh had to drive home with them secured to the roof with several ropes.