I'm going to try the heart shaped bustier cake for a friends bridal shower.
I'm new to making cakes. My question is for the 3-D boobies do I buy the
small round ball pan? Or does someone have a simple way, maybe a bowl or something? Thanks for your suggestions.

*Anything* that is the shape and metal can be used to bake in.
Of course, there is also oven safe custard cups etc that are not metal those c ould also be used.
If nothing else, you could use standard size cupcakes & shape them slightly.