It is useless to buy a natural pearl and it is very difficult to find one. Even if you find someone selling natural pearl, I am sure it won’t be; because it is absolutely rare and maybe sold in auctions only.
There is almost no difference in natural and cultured pearl. Cultured pearl is not a synthetic pearl, it is also a natural pearl but it has been cultured i.e. grown with an oyster (organic molluscs that live in marine) with intervention of man. I think you are still unclear about natural and cultured pearl so I will explain you in simple language so that you will be able to better understand its differences.
Oyster is a mollusc which lives in marine (sea/ocean). When a dust enters into the mouth of molluscs accidently (naturally) without intervention of a man and after sometime a layer after layer is created on the dust forming a natural pearl. However, when that dust is put intentionally by a man inside the mouth of molluscs and then a pearl is formed it is known as cultured pearl. There are cultured pearl farms in different parts of the world; one of the best pearl culture farming is done in Japan. That is why one can get cultured pearl (or you can say natural pearl but made with intervention of a man) in all possible fancy colors. However, for astrological purpose it is suggested to use white pearl only set in sterling silver.