These animals aren't like clownfish which are easy to identify, stock and sell. Any cephalopod is a specialty item and often when a retailer has one in stock it is by accident and they are unsure of not only what species it is, but what ocean it came from!
Any time you order from a retailer you're rolling the dice. Ultimately, the only way to know you're getting a bimac is to obtain one directly from a hobbyist who has bred them. As Monty mentioned there is someone working on this now, but there are no guarantees in life and it might not come to pass. This hobby requires considerable patience--it took me two years to find a bimac.
Dan