Rhodium, I know there is no elemental Br in weedkiller, or in anything else I could think of that is on sale to the public.
I think the idea of trying to convert to NaBr would bee an interesting thing to try, after conversion to the group I bromide acidification with conc. H2SO4 should release the Br yes?
I came up with this because there is no pool supplier that I know of anywhere near my area, I don't know as the UK is known for them really, not warm enough for private pools, and I thought some bees might also bee able to benefit from this.
And besides it hadn't been covered in TFSE, so I suppose it's another possible source although I can't do any testing because I don't have access to my lab at the moment.
One other thing, it seems diquat is strongly attracted to clays and dirt because of it's charge, so much so, that it cannot bee used in muddy water, maybe this would bee a way to seperate out the diquat from solution, using diatomaceous clay or fullers earth etc.