Look in your local phone book for Casting Companies located in your area.
Call the casting company and ask if their looking for film extras, or if they have a database that you can sign up for. Databases usually ask for general info and a few particulars, like hair color, clothing sizes, experience, and any special skills you may have that would qualify you for a more specific extra's role.
(If they charge a fee, move on. Extras should be able to sign up for free)
Check craiglist.com under TV/film or talent. There are usually listings for film extras needed in and around your area.
Whenever you see something of interest, apply immediately. Follow directions to the letter.
Also search Meet Up Groups. There are Film and Actors Meet Ups that also list postings for film extras, stand ins, movie doubles, etc.
Become active in local theater groups. It's good practice and will also give you contacts that may be able to provide valuable information about upcoming roles.
Good luck! And, break a leg.........