You could buy a premade sphere and then spray paint it? Why make a sphere when you could easily modify a pre-made one? There are a lot of solid plastic ball toys at petshops, for example. Or you could even buy a weight ball like this: http://www.ymoshop.com/index.php?main_page=product_info&cPath=93&products_id=10592. Or buy a bomb shaped candle at an online novelty store.