There is a large, APA-accredited internship program at the University of Maryland School of Medicine / VA Maryland Health Care System Consortium. At this consortium, there is a formal ACT rotation in the area of PTSD/Substance Abuse dual diagnosis and there are actually several psychologists who could provide ACT supervision; an intern could thus spend a good portion of his/her training year honing ACT skills.
Please visit: https://www.va.gov/maryland-health-care/programs/mental-health-clinical-center-psychology-training-program/ for up-to-date information.