If you are a supply teacher looking for work, you may want to consider registering with an agency. Supply teaching agencies will often have posts available for you to start immediately and once you are on their books, they can put you forward for any new teaching work that comes in.
Most teaching agencies do not charge you to register with them but will take a small percentage of your daily wage once they have found you work. This does put some people off, but the extra amount of work you are likely to get from using an agency will easily more than cover their fees.
To decide which agency to use, I would recommend trying to obtain reviews and also statistics off the teaching agency such as how many job vacancies they currently have on their books. Also confirm any fees up front to avoid disputes further down the line.

