As others have said, foam core board is available at Walmart, office supply stores, and craft stores. It’s basically rigid foam sandwiched between an outer layer of paper or cardboard. The outer surface is sort of like poster board so you can write on it or glue things to it easily. It’s lightweight but rigid so it doesn’t bend. It comes in different thicknesses but 1/4″ is common. Walmart lists a 20″ x 30″ sheet for $1.97.
By the way, if you also want a globe now is the time to get one. Many stores only stock them during the back to school season so they will be getting into short supply soon.
Here’s one more idea for maps. Put large maps on the kitchen table then cover the whole table top with a large sheet of thin plexiglass. 