I disagree with the height being all that important. Most boa breeders will keep their adult females in 4x2x1 cages. Of course if you get a monster (which is rare) you will probably have to go to a 6 foot cage.
Boas also have litters instead of clutches but not sure what time of the year is most common for them to be available. Jimisnakes, VPI, and Rio Bravo Reptiles come to mind when thinking of good boa breeders although there are tons more out there.