The approach to seeking mental help for an adult depends on what the issues he is experiencing are. A good place to start is to get a referral from your primary care physician. They will talk to you about your son's symptoms and recommend a specialist based on family history, specific manifestations, and other symptoms. Here's a link that may help you find specialists in your area : http://www.netwellness.org/healthtopics/mentalhealth/mentalhelp.cfm
You should discuss what you find with your primary care physician to ensure both proper treatment and that the course of treatment is covered by your insurance.
I would go online and look. Your state should have low or no-cost programs that offer assistance, if not, there should be a source that can point you in the right direction. Look under mental health. Good luck!