Pretty much any EM residency offers a "sub-i" in EM, whether they title it as such or not. Places that come to mind: Denver, LA county, Indianapolis, BIDMC, MGH/BWH, NY Presbyterian, Christiana in Delaware, Georgetown, SUNY Downstate, UCLA Harbor, SUNY Buffalo, Cincinnati, and I could go on for days.
Like I said - pretty much any EM residency will offer one through their affiliated medical school.
It may not be specifically listed as a sub-i or acting internship but under the elective section. Contact programs that you may be interested or that offer a different perspective from your home program (i.e., county, community).
Many schools require 4th year electives in EM and some programs leave the sub-i designation to more traditional fields like medicine/surg.