Have you looked at Animal Plastics cages? They are very high quality and reasonably priced.

Here is their website: http://www.animalplastics.com/

I can answer any questions you have on AP cages

I'm not knowledgable with BRB's but I do know that Emerald Tree Boas should have a 3 x 2 x 2 foot cage or a 4 x 2 x 2 foot cage.

This picture showes 2 AP cages stacked (a 4 x 2 x 1 foot on top and a 4 x 2 x 2 foot on the bottom)