In the US, The Dollar Tree is the typical merchant of choice, but elsewhere it varies -- better to get info from someone local. Until we know where, the forum member near you can't help with local information.
This is the stuff that Flite Test uses and there is a very good reason why. Like Crafty Dan asked, where do you live? If you live outside of the United States, then Dollar Tree probably isn't the best choice for you because of the cost of shipping. Plus Foam Board is easily damaged during shipping.
While 3 times the cost ($3/sheet), if you want to stretch your plans or scratch build something bigger without added seams, you can pick up 40"x32" at Dick Blick or some 40"x30" at Staples. It's expensive compared to Dollar General; but, if you need big... it's an available option.