This may be too late, but I have analyzed a couple of urea samples commercially available as laboratory reagents. Arndt Schimmelmann also has a few available. Not including ones that are artificially enriched, the samples tend to be on the lighter end of the biological carbon isotope spectrum. The one available from Dr. Schimmelmann has a natural delta 13C of -34.13 permil; the two I've analyzed are -32.8 and -48 permil. I still have some available if you need something for comparison.