What they said about swaps and auctions. It is fun but you may get to know a lot more than you expected about chicken health care, which local vets treat chickens, and methods of euthanasia. Not just for the one you brung home but for the others who catch stuff from it.
Plus, every time you add a new chicken or two to the flock, all sorts of dinosaur-era Social Interactions go on, using beaks, often rather vigorously and distressingly and sometimes for a long time.
It is really a lot simpler and more peaceful to get them all at once, IMHO.
If you want marans or dark eggs, be aware that hatchery birds are quite likely NOT to lay a dark egg. Even birds from a breeder may not do so consistantly (it takes constant hatching-out of large numbers of chicks and culling down to only the darkest layers to keep a line laying reasonably dark eggs). Not trying to discourage you from BCM (or other marans, or welsumers, or anything else like that), just want to make sure you realize this fact.
Personally, in your shoes I think I would order from My Pet Chicken since they have the breeds you want, get a couple few of each breed (and/or combine orders with someone else in your area) and sell the extras. This way also ensures you DO get a pullet of each breed - remember that "guarantee" is only 90-95% and is only a pledge to refund your purchase money if you have too high a %ge of cockerels, it is NOT a literal guarantee that if you order a pullet chick it will definitely for sure be a pullet.
Good luck, have fun,
Pat