That's an interesting theory, and one I'd never really considered.
It's got me wondering whether or not the belts might have somehow, directly or indirectly, come from the Erlking and his court. The FBI agents were, in a way, hunters, and those belts definitely gave them more hunting skills...
They still could have been delivered by Kringle, or through some other means (especially if they were tainted by Nemesis after they left the court.)