i spent about a month calling around trying to find one too. marinedepotlive.com is supposed to have two baby tank raised bimacs but they ended up sending me a different octo by mistake. i presume they still have the octos that (i was told) are bimacs. Nevertheless, through this whole process i have learned that basically you cant believe anything any seller or dealer tells you.
the odds of a dealer listing a specific species of octopus, let alone it being accurate, are slim to none. you have to expect that most will just be listed as miscellaneous, indo-pacific, etc. the forums here are much better at checking ID's anyhow
Fishkid. Keep an eye on the "Octopus Availability" thread, as that is the main place for this information. Unfortunately, getting a particular species from a supplier is a gamble in most cases. If you want a bimac, the best bet is wait for someone who breeds them-- Zyan Silver's dad took over his "octopus farm," but hasn't given us an update lately. There seem to be a lot of aculeatus, hummelincki, and briareus available the last month or two, but most dealers have not been representing the species accurately-- someone seems to have invented the "pigmy bimac" term, which is usually a misnomer for hummelincki, except when it's not. There are a few divers who are good at IDing briareus and mercatoris, so if you buy from one of the collectors in Florida who's known to be able to ID these species, you'll know more about what you'll get.
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.