Hmm. Can't help too much except to maybe give you some store ideas. I'm assuming that if you're on the North Shore you're probably using Curious Creatures? If not they might be worth a shot but the last time I was in there I was very disappointed with the condition of their tanks.
Tropic Isle Aquarium is a pretty large store with a lot of selection. I'm not sure how stocked the are on live bearers though. They're in Framingham right off of Rt 9.
Lovely Pets in Quincy is another great shop. Quite the haul for you most likely but worth a call if closer stores don't have them.
Aquatic InXteriors in Canton is a good shop. Not huge but very nice. The staff there is approachable and helpful too. they may be able to get some endlers for you if they don't already have them.
Boston Tropical Fish and Reptile in Cambridge is another small but pretty helpful shop. They carry some oddballs too. Worth a call.
I would certainly call these places first but Tropic Isle is worth a visit anyway if you have the time.
Tom