These sheets should be non-transparent, pure white, 4-6mil in thickness, and relatively stiff (not like that used for plastic bags, but rather like mylar sheet).
Most hobby shops should be able to supply you with this type of material. If they do not have it then they should be able to locate a supplier for you or at least get you a catalog that would allow you to purchase modeling materials. The link below is for a company that sells many plastic sheets and scaled plastic structural members for railroad models and that type of thing. Send for that catalog.
