Interesting question. It is, of course, a very difficult area to research. I would look for findings from anonymous surveys, possibly online surveys, as they are best at providing more accurate/valid data on stigmatized topics such as drugs, sex, mental problems Tourangeau, R., & Yan, T. (2007). Sensitive questions in surveys. Psychological Bulletin, 133(5), 859-883. doi: 10.1037/0033-2909.133.5.859
A couple of paper I saw looking at drug dealers, but unfortunately not providing data on psychiatric symptoms, were:
Semple, S. J., Strathdee, S. A., Volkmann, T., Zians, J., & Patterson, T. L. (2011). “High On My Own Supply”: Correlates of Drug Dealing among Heterosexually-identified Methamphetamine Users. The American Journal on Addictions / American Academy of Psychiatrists in Alcoholism and Addictions, 20(6), 516–524. http://doi.org/10.1111/j.1521-0391.2011.00173.x
Semple, S. J., Strathdee, S. A., Zians, J., & Patterson, T. L. (2013). Correlates of Drug Dealing in Female Methamphetamine Users. Journal of Urban Health : Bulletin of the New York Academy of Medicine, 90(3), 529–541. http://doi.org/10.1007/s11524-012-9748-9
You might try using Scopus and checking who cited these papers.