CYNTHIANA, Ky.—Tony Ockerman, a fourth-generation farmer hoping to diversify his crops by adding hemp, said his first harvest this year turned out well after plenty of trial and error. But he is struggling in one key respect: finding buyers.

Hoping to capitalize on surging demand for cannabidiol, or CBD—a derivative of hemp or marijuana that proponents say has numerous medical benefits—Mr. Ockerman invested about $100,000 to grow 30 acres of the plant. Unlike some farmers who secured contracts with buyers ahead of the growing...