Try searching for promo companies in Google. Some do different things though (I used to be with two, one did cosmetics/alcohol and the other event hostessing), so check out their websites for details before you apply. They'll call you in for an interview, then if you're successful you give your availabilities and they'll call up to book you for a client and give you details on the products you promote.
What sort of sqruiking do you want to do? Microphone stuff, sampling, leafleting...?