For Kansas City Barbecue Society (KCBS) style barbeque competition events, click on the link below. This organization holds contest in almost every state, Puerto Rico, a few in Canada and Europe. It has a search filter so you can search the events by either Name; Date; Place; or State.
A lot depends on State and County health law that govern whether a competitor can handout or sell his food. More often than not, there are vendors that travel the circuit who sell barbeque; and even some vendors compete.KCBS Competition Event
Also check regionally. Many regions have local barbeque organizations that hold events. Local barbeque organizations often piggyback onto a KCBS event, and will hold a Backyard Competition at the same site, either the day before or after the KCBS event.