I purchased a pastel banana BP for my daughter this past summer. We are fortunate enough to have a reptile specialty store near us, so finding equipment, supplies, livestock and advice was pretty easy. They do their own breeding and do consignment sales for local breeders.
Our local store will give you a normal BP for free if you buy the set up from them. We paid more than I want my wife to know for our pastel banana boi, but not nearly as much as we did for his habitat. We also made a few costly mistakes along the way. We purchased an Exo Terra glass cage which we will soon be replacing with an AP T10. We also purchased a cheap thermostat that we soon replaced with a Herpstat (so we could could have worry free sleep).
My point is that before you start worrying about purchasing your snake, you should start pricing your habitat. We had ours set up and stable for several weeks before we brought home our BP.