There are about a million and a half shops at the ren faire, and they all have stuff for sale. If you've never worked faire before, you don't want to rent one of those booths. You want to work for one.
Two years ago I answered an ad on Craigslist, and got myself hired by a jewelry booth. We all got on great, and they've wanted me back every year. Let me say I was really lucky to get hired by a reputable company. The pay is ok and always on time. Some will try to pay you in goods.... it's nice if you like what they sell, but it's unprofessional, illegal, and should be a big warning sign that this may go badly. What OTHER unusual demands might they have? What safety precautions might they ignore? So check the local Craigslist the month before a faire, but screen the posts VERY carefully.
If you are a fan of a particular company or companies, email them in the weeks before a faire. Tell them what a fan you are and that you'd like to work for them. This targeted approach works well because people are flattered that you are already familiar with and like their goods. If they have the people they need, they'll tell you so. But they might also add that so-and-so could use someone.
Go to the renfaire's website in the month before the faire. Run down the list of links, and check out the sellers website. If they need a person for a particular faire, it may be mentioned there. Again, email or call.
Lastly, you can go on the opening day of the faire and ask around to see if they need anyone. Go in GARB. Go EARLY. Hit the booths before the noon rush comes. If you look well-dressed and reliable your chances may be pretty good. After all, they will be needing someone pretty badly at this point.
Hopefully this will help some of you achieve your dream ren faire job. It will never make you rich, but every day brings something new and unexpected. Good luck!