All of the over the counter "bleaching" products that contain hydroquinone are limited to a 2.0% concentration in the United States. Prescription creams are usually 4% and dermatologists will sometimes specially compound ones that are higher strength. 

Still, the over the counter products are worth a try. You might try the Glytone Bleaching Clarifying Gel or the Cream or the Clinician Complex 6% Bleaching Cream (oddly the "6%" still has 2% hydroquinone).  Just remember, some people are irritated by any hydroquinone and will get worse using it.  If that happens, you must stop using it.  Here is some more info on treating this problem.   Dr. I