Step 1: Go to the chains presentations or seminars they offer. (if they offer them at your school)
Step 2: Find who the Regional Recruiters or Pharmacy Supervisors are for the chain you plan to apply.
Step 3: Go to some of the stores around you and ask the pharmacist you want to intern there and they usually will give you the pharmacy supervisors or recruiters #.
I got a CVS internship through the regional recruiter who came to our school to give us a presentation about CVS. I approached him after the presentation and told him i wanted to work for his company. Within a week i had a Supervisor call me for an interview.
I got an internship with Meijer (which is pretty exclusive in this area of the town) by emailing the clinical coordinator a few times to ask if any positions were open until i heard back when a position opened.
Just my personal experience
Good Luck
Edit: I just re read your post and you can not become an intern prior to starting pharmacy school and obtaining your intern license from the state. Wait until then and then you can look for an internship.