Sounds like a neat project. Jim is first mentioned in chapter eight, living on Jackson Island when Huck runs into him. As for the poster itself, there would be a reward posted. The actual price for a returned slave would largely depend on where exactly Jim would be captured. There would be a description of Jim, any other names he would be known under, perhaps a brief detailing of his clothing, as well as a signed statement by Jim's owners. There might also be a seal or some offical marking. I've only seen those on a few runaway slave wanted posters, so I am not sure if those were required by law or not. Jim's owner (Miss Watson) would have to have a statement ready, signifying her intentions, i.e., should Jim be held in a jail or just brought straight back to her, etc. I've included the summary. That should help get you started.