Must have missed this pne.
There are really two types of "internships" that come into mind - the first one is your standard free "monkey" who will do the mindless work for you and the other being a path to a the person building their skills and having something on their resume.
For the free / cheap labour try the facebook groups, high school placement offices and the university sites like CEDAR:
Careers & Placement
For the more sophisticated intern (long term, not just a summer job) you'd have to work with a uni or two to develop a curriculum or process that you're going to put the intern through - i.e. similar to a course that they would get credit for. Many of the degree and advanced degree programs have practical work experience programs that the students need to undertake.
There you'd have to work with a uni or two to understand what their requirements are from internship programs - i.e. they'll recommend your program to their students, the students work part time, optionally make money and earn a credit or two.
On a side note - I detest seeing "social media intern" on CVs from entry level grads - basically meant that they spent their time at a company scraping sites, spamming and creating noise .. the kids seem to think that this qualifies them for six figure jobs.