I think a good amount of big band is being preserved by the dancers. I was at a dance this Friday - with a 9 piece live band. Great musicians. 200+ sweaty 20-somethings eating it up. It is not being re-issued as quickly as other genres of music for sure, but there are countless titles available.
Sidney Bechet, Andy Kirk, Lionel Hampton, Leo Watson, Hot Lips Page, Erskine Hawkins, and Charlie Barnet are some lesser known big band leaders worth looking into. Honestly there are too many to list. I transfer my physical media (most being 78's and 33's) to DSD. 32 days of continuous big band and jazz music from my library, according to the JRiver counter.