Brushy Mountain is currently the only US distributor* of EPA 'labeled and approved' oxalic acid for use as a varroa pesticide. By Federal law, pesticides may only be sold and used out of a container that is labeled with a complete use and dosage label that references the correct usage of that pesticide to control the target pest.
In addition to Brushy Mountain selling that EPA labeled product directly, their dealers also sell the Brushy Mountain EPA labeled oxalic acid (OA). Also, Brushy Mountain wholesales its EPA labeled OA to some of its competitors in the bee business. For instance, Dadant sells Brushy Mtn EPA labeled OA.
However, what Mann Lake is selling is not labeled as a pesticide, and not legal to use as a varroa control. But that does not seem to stop Mann Lake from selling oxalic acid labeled as wood bleach.
*(in this sense, "distributor" is meant as "manufacturer", although Brushy does not actually own a manufacturing plant producing oxalic acid.)