Where did you end up getting the Magnesium? I grew weary myself of trying to source enough so I switched to Walmart and Canadian Tire about 6 months ago. Bath salts for the sulfate and sidewalk ice melter for the chloride (I did research what brands people were using.) So far I haven't noticed anything bad but a part of me keeps thinking "this is bath salts and sidewalk ice melter".
So I'm curious about alternatives still.