Depends on where you live. In parts of Europe, Schleiper (www.schleiper.be) carries large sheets (like 2x3 feet) of it as well as smaller sections of various thicknesses. In the States, most large hobby stores have some selection of it. Also try art supply stores. If you live in a city with an architectural school, then there will definitely be a store somewhere that has plasticard, foamcore and other goodies that architecture student use for their projects. Ask the prof. or a student. Hope that helps.
As for making it round... not sure what you mean. Cutting circles out of it? Or plastic rods? Plastic rod is also usually sold in the places I mentioned.