In my opinion, NO boa should ever be kept in something as small as a 40gal breeder tank. My adult female BP is in a 40gal breeder and it's even somewhat cramped for her.
www.pvccages.com is another place to find a good cage for a reasonable price. Jim's cages are really nice and well made. You can get a 4x2x1 pvc cage with a 40 watt RHP for heat for only $270 shipped. A 4x2x1 should be fine for the average sized male BCI and would be fine for any of the dwarf localities.
Another thought is buy a baby. A baby can start out in a 27-41qt tub or a 20gal long tank and grow into a good sized cage. You'd have plenty of time to save up for the cage it would need as an adult.
Be patient and save your money for the right enclosure and you and your boa will both be happier.






