I second the craigslist thing...or a model networking site (Model Mayhem), but Amoona's right about most of them being TFCD...especially on sites like those.
I have been fortunate to only have good experiences, but I am anal like that. You want to make sure that you find a show that is organized. If you a responding to an ad posted by the designer, you need to get all of the details-number of models, looks, compensation, call time, other MUAs, etc.
Ideally the ad will be posted a key MUA who is responsible for everything as far as the makeup is concerned. They will create the look, hire the MUAs, connect straight with the designer, and basically be your boss & your advocate. They make sure that there will be enough MUAs to cover the amount of models in the allotted time and that in the end it if all, that you get your pictures/$.