coombesy - HBr is the standard salt form of DXM in most OTC medicines. It's not very expensive even in OTC, by-the-capsule form, and like most things it's even cheaper if you buy it in bulk.
BTW, fun fact: because of DXM's use as a cough suppressant, and the fact that it was in the process of replacing low dose codeine as the OTC cough suppressant of choice when the Controlled Substances Act was passed, it is explicitly exempted from scheduling, meaning Congress would have to amend the legislation to allow it to be scheduled in the future. It's not only unscheduled, it's explicitly clarified that, for the past 40 years, the federal government has not seriously entertained the notion of scheduling it. That's probably another reason it's so cheap compared to many RCs - there's no risk of it being considered an analog, no need to pretend it's not for consumption, etc. as it's an approved, unscheduled OTC med.