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BlueMist Vaping Supplies
They sell 36 mg/ml, just cut it 50% with vg or pg and you will have 24 mg/ml.
(20 ml of the nic liquid + 10 ml of vg or pg = 30 ml of 24 mg/ml)
Plain vg is available in many pharmacies, called glycerin usp (skin protectant), near the bandaids. 6 ounce clear plastic bottle is around $5. Ingredients should only be glycerin 99.5% or higher, maybe followed by (anhydrous).
Pg usp is available at some tractor supply stores, or just buy a small bottle of it with the nic liquid.