you can use a large pyrex bowl for the belly or 1/2 of the large ball pan, and for the boobies you can use the small pyrex bowl or the mini wonder molds, they look realistic... i secretly think wilton made them for naughty cakes 
if you look in my photos there is a green belly cake and I used the small ball pan for the belly and mini wonder molds for the boobs...hth gl
i've never made one but maybe this thread will help.
http://forum.cakecentral.com/belly-cake-ftopicp-5963721.html#5963721
I did a belly cake for a baby shower and I made a 1/2 sheet cake and for the belly i used the soccer ball cake pan and for the boob's I used a small foam ball and cut it in half, making it flat on the bottoms and rounded on the tops. I covered them with fondant dress, turned out so cute!! If you check a craft store you can find foam balls you could cut in 1/2 for the belly if you don't have the soccer pan. Hope this helps, good luck!
same here, the sports ball pan and the mini ball pan
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