J.J.
I know that there is going to be one at Udall Park on Saturday and a few other places in the area. I think it was channel 4 that had the schedule for the vaccinatioins.
This is what I found online.
Free MMR Immunization Clinics - Saturday May 3, 2008
Saturday May 3, 2008 - Kino Sport's Complex Soccer Fields (Directly behind UPH Kino Hospital) 2500 East Ajo Way 8:00AM - 3:00PM (Free Clinic)
Saturday, May 3, 2008 - UDALL Park 7290 E. Tanque Verde Rd. 8:00AM - 3:00PM (Free Clinic)
Saturday, May 3, 2008 - Rillito Racetrack Park 4698 N. 1st Avenue 8:00AM - 3:00PM (Free Clinic)
I would also just call your pediatrition to find out if they do it or call Pima county health department.