Reptile Basics is a great place to get cages, no contest.

However, I think your snake is still too small. I have my 18mth old girl in the 20long tank she's always been in and she's only just starting to look cramped. (I got her a 40g breeder tank to move into, since she hates tubs and I can't afford an RBI). She's around 600g and small for her age as well, so she won't be moving for a while.
The general rule is to have an enclosures length+width equal to the total length of the snake. But BPs often like things a little more snug than most snakes.

Also, only the BIGGEST of BPs should go into that 2x4. It's HUGE! A 2x3 RBI cage is just perfect for most adults.

I would wait until yours is at least around 1.5-2 years old and of a good size before considering the 'adult' home.