Without buying new, these are out there, but not that easy to find. Keep checking online sites and music stores, 4 valve BBb sousaphones show up periodically. I bet that there are a number of these in school districts in the back of a band room closet, unused because they need work or are too heavy for marching, so you might check with some of the older schools/colleges in your area.
I bought my 40K for about $1400 on eBay within the past year. They show up there periodically. More 4 valve Eb sousaphone appear to have been made, or at least I see more Eb's with 4 valves than BBbs. Even so, those are also rare these days. Unfortunately I have yet to find a 4 valve Eb sousaphone that plays well for me. I have a Conn 28K and 4 valve King raincatcher, both of which are just mediocre players. OTOH, the 40K is a great sounding and playing horn.
