I do supplement urea yes, but only recently so it's a little soon to say. It seems I can find it in chemical supplies stores, under the heading USP grade or ultra pure urea.
More protein can increase urea if you can metabolize the protein. My guess is that people here have various metabolic circumstances that might make it harder to metabolize a lot of protein.
Another way to increase urea is the time-tested way of drinking urine. I don't know much about this other than the gold standard: start slow and gradually increase looking for signs of excess in nausea, headache or fever.