where do I find the bowl pan for making the baby bum cake?

Not sure about a "bowl pan" but if you're looking for something round there's the Wilton ball pan sold at Michael's and other places that sell Wilton stuff. That might be too round for what you're doing though.

I use a pyrex bowl for all cakes needing a 'rounded' shape and then just carve into the desired specs you're trying to achieve.

Takes at least an hour to bake, with the bowl being liberally greased and floured.

HTH
Kat

I have a set of aluminum mixing bowls. I use the largest for the bump.