I don't think fitting a busty woman is an issue. Fitting a slim very busty woman maybe more of a problem.
Try off the rack, and see what you think? If it's really tight in the fitting room, then it's going to be uncomfortable to dive in, but keep in mind wetsuits are suppose to fit snug or a bit tight. You want to try and get a suit that is snug, but not uncomfortable. It should be snug at the seals, and fit like a glove if possible.
The stretch material really does expand well. Do they make a neo sports bra or something to bundle the chest, like gymnasts use? Maybe that is not needed..but could help.
Hyperstretch, or superstretch or XXXXstretch material is your best friend! I'd exclude anything that is not made with stretch. Different manufactures have different names for their stretch suits. I have a Tilos 3mm stretch suit, and they are very comfortable.
If you can not find an off the rack suit (I personally think you should be able to), than a custom suit is certainly an option. They tend to run maybe $250~400 for a 3mm depending on a lot of factors. That is a bit more than the average online wetsuit cost, and it will take a week at LEAST to have it made, likely closer to 3~4 weeks, but talk to some manufactures.
Bet you did not expect all these responses, but if you mix boobs with diving on Scubaboard, the response is awesome!