There are folks on here that sell baby Bimac's occasionally. Some have eggs right now, the babies won't be ready for a couple of months probably, and after that they will probably have a few left for when your tank is up and ready. You shouldn't worry about that until your tank has been up and running for several months though, it will just make you more eager and make it harder to do the responsible thing.
It's very unlikely that you will find Bimacs available at a LFS, but it's not unheard of. You might ask your LFS if they can special order any octopuses, and they should be able to tell you how much they charge for them. Stay away from "Zebra", Mimic, and Wonderpus though, for multiple reasons. When the time is right, you might also check out some of the online saltwater retail stores like liveaquaria.com and saltwaterfish.com