Blacklight reactive dye for finding coolant leak?

I am loosing coolant in my e-150 van. It doesn't overheat unless the coolant level gets low. It used up over a gallon in three days and 300 miles. But there's no moisture in the engine compartment or any poolede up underneath. I can smell hot coolant when I get out but not inside. Theres none coming out the vents and my windows don't get steamy inside. I fchanged my oil and it looked fine no milk or anything. I've heard of using uv dye and a blacklight to find the leak but I cant find a brand or a store that carries it. anyone done this or know where I can find the dye?