Quite a lot of LFS carry Brightwell, but what I do when I want something is just start making phone calls down this list (based on who is closest to me, radiating outwards):
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If you were in a pinch I can squirt some into a little cup for you... but I'm near I-20 and 161.
I only use it in my temporary macroalgae grow out tank (along with NeoNitrate & ChaetoGrow)