Sintra is an expanded PVC foam, so no it is not polystyrene, very different materials. Sintra is easily shaped with a heat gun. Styrene is great for build ups and vacumforming.
Both materials can be found at your local sign shop or plastic distributor. The nice lady at my local sigh shop sold me some scrap styrene at a very reasonable price. I even got a 4'X8' sheet of 6mm Sintra for $10, she couldn't use it and wanted to get rid of it. It never hurts to ask.
Alan