As a legal matter, it IS pefectly legal for an American company to buy insurance covering risks involved in otherwise lawful commerce. (If you were to be buying cigars from Cuba, or oil from Iraq, or selling arms to North Korea, it is likely that as the transactions are illegal, insurance against loss from engaging in the transactions would be against public policy.)
You can buy that insurance from any US based insurance company that provides it, or from a foreign insurance company. Ask your broker or if you don't have a big one, a firm like Marsh or AON.