Sculpy firms with heat of the oven to a hard clay/plastic, it therefore is not a good filler for Sintra...since heat allows you to shape it and cold firms it from it's formable state. Bondo firms with time (around 5 minutes if 2-part...usually less), and if not careful, you will ruin many hours of good work...it can be unforgiving.
Sculpy is good for greeblies and shoulder studs. Filler putty smoothes imperfections.
You can get Sculpy or another brand of "oven clay" at most Wal-marts in the crafts section. They won't have many colors in stock, though. If you don't want to paint the dried sculpy and need it to be a certain color, try a Hobby Lobby, Michaels, JoAnn's fabrics or other similar large craft store as they usually have a huge selection of different colors.