Schools usually require principle's permission before things are hung up, and in offices, it's a good idea to check with a superior first. For those who have slightly larger budgets, posters can be made up and copied in color by Mailboxes or Kinkos. One way of doing this is coloring in markers or in water color paint the poster and having it copied. Beyond flyers and posters is advertising. Obviously this costs money, but some people will do it for free.
Most local papers have a listing for such events, and cable access channels also list local events.
Also check if your town has a web page listing current events. Proffesional theatres obviously have more money and can advertise in papers. Advertising Equally important to filling the show up is selling advertising space in your show program. There are two ways of selling ads. First is directly selling them by gowing to local retailers and offering them space.