Hi Debbie
Why is a natural pearl preferred? Even imperfect ones can be quite expensive. Natural pearls occur in the wild with no human intervention. The natural pearl business has been dead for about 100 years, supplanted by cultured pearls. With few exceptions it is quite difficult to find any natural pearls newer than that.
The island of Bahraini sells guaranteed, certified, natural pearls. Any other than natural pearls are illegal, so you know what you are getting.
On the other hand Chinese cultured freshwater pearls (CFWP) are solid nacre like wild pearls. (they don't have a round mother of pearl bead nucleus covered by nacre to a thickness of up to 1 or 2 MM) and the the imperfect shapes shape occur often.
I am wondering if "imperfect shape" means not quite round or perhaps you are thinking baroque? The following picture is one of several from this thread
LINK
The photos on that page are all strands of pearls meant to be turned into jewelry. You can buy a single large baroque CFWP also. I'll look for one for you.