Just find a local psychologist or psychiatrist. You probably want to look for one who's experienced in diagnosing autism spectrum disorders. Also the rates of psychologists and psychiatrists will vary widely, and diagnostic testing can sometimes get expensive. Depending on whom you see, the cost of diagnosis may or may not be covered in part by an insurance plan. In my case, half of the cost is covered by my graduate student insurance since I was examined at the Penn State campus psychology clinic. However, this is not the norm as many insurance plans do not cover diagnosis. Good luck.
-Scott