The most important part of buying sorghum is... to make sure it's pure sorghum, and not sorghum-flavored corn syrup. (Kind of like Aunt Jemima's "Maple Syrup"

)
A number of years ago, I stopped at Lambert's Restaurant ("The Home of Throwed Rolls"... never mind) in Sikeston, MO, in the southeast corner of the state. Leaving after lunch, I spotted a half-gallon tin of Sorghum. This is the right area for sorghum, so I bought it, since I'm a long-time sorghum fan. Got home, read the label and, sure enough, it was corn syrup with a little sorghum flavor. Threw it out.
My last real-sorghum score was a specialty food store in a small town in west-central Pennsylvania; before that, a farmstand in the small town of Gnaw Bone, in central Indiana.
So maybe you better try the Internet!
Suburban gourmand