Animal Plastics, ConstrictorsNW, BoaMaster, and BoaPhile all make decent-quality enclosures that will work. I'd recommend one at least 48" wide x 30" deep.

Since your guy is fairly small and you have some time before you have to get a larger enclosure, keep an eye on Craigslist, local FB reptile classifieds, forums, etc. Sometimes people sell off the larger enclosures at a steal of a price, because the cost to ship them is prohibitive. As an example I picked up four 48"x30" Boamasters with heat already installed for $50 each from a guy getting out of boas, because I was willing to go pick them up and paid cash. Obviously I had to clean and sanitize them once I got them home, but that didn't take long at all.