I would start with small-ish, local shops, like flower shops, beauty salons, bakeries, used car places, etc. Places that are probably run by an older individual that doesn't currently do Facebook advertising. Don't ask them for an 'internship', because that implies something formal that would involve them having to do something.

Just go into the shop and ask for the owner, and tell them something like, "Hi, I run a program in which I do Facebook advertising for free for local companies. I help design Facebook advertisements, and then I help post and manage the ads to get them to benefit your business. Would you like to op in to the program? It's my way of contributing to the local community. I live up on xyz road"

If possible, before you visit:

1) Prepare a well made mock up of what you envision the advertisement would look like (use their logo / slogan from their webpage, etc.). You could have this ready to show to the shop owner.

2) Have already self-deployed the ad on Facebook, and have some data on its effectiveness in driving customers to the companies website. In other words, use your own money to have the ad online for ~ 1 week, and see how many click throughs you're able to get, and the $/click. Tell the owner that, in order to help companies in the local area you've already deployed ads for several local business for a couple days. Show them the number of potential customers that have clicked it, and the $ / click.

3) See if you can get any sort of support from a local community board. Doesn't have to be financial, it could be as little as an email response from them saying they like the idea. It will increase your ability to convince shop owners that you're not a scam artists or anything.

As you build up your experience and successes you could start reaching out to larger, non-local companies.