Urea is a component of urine. I haven’t looked it up, but I suspect the separation isn’t hard - for a chemist. If it only comes in contact with exhaust one can’t hurt the engine by using the wrong stuff - though it could stink. Urea is the solid with the highest concentration of nitrogen, making it the cheapest fertilizer. It’s for sale on eBay.
I use a urea cream to treat calluses.