My dad is an alcoholic and gets violent when he drinks. Where can I get help?

ACOA. Adult children of alcoholics (acoa) is a branch of aa and can found locally at www.Adultchildren.Org they serve as a place to talk about your experience with others who have had similar experiences. If you still live at home with your father you may want to get out of the house when he's drinking. Consider therapy for yourself, having an alcoholic parent can be extremely stressful.
Seek assistance. Those that can help with alcoholism include: chemical dependency unit (cdu) at your local hospital or at a free standing cdu, clergy, counselors or therapists, physicians who specialize in addiction, and even friends or family members. However, these latter two should be done cautiously as well meaning friends or family without experience may actually cause more harm than good.
Counseling for you. First get supportive counseling for yourself. You can sort through the issues and make some choices about what to do. You can encourage your dad to get help to change, but ultimately you can only change yourself and your response to the situation. Alanon might help also. You could ask him to attend aa or counseling with you. I wish you well. This is a tough situation.