Know your talent budget. Determine how much you can pay for actors. Most of the big name stars do an independent film for a reduced rate if they strongly believe in the subject matter or like the role.
Send your script to anyone willing to read it. If you hire a casting director, he does this for you. Casting directors have established a relationship with talent agencies and management companies, which might help you find a more recognized actor. If you are casting on your own, this requires a lot of work.
Contact your local college theater or film department. Many students studying to be actors look for the experience of independent films. Hiring these students costs a lot less and many of them are talented.
Advertise in trade magazines and local newspapers--especially for extras, day players and background actors. Many of the actors you get come from community theaters and college programs.
Ask friends or family members to play a part. Even if they don't have a lot of talent, it allows them to have some fun. For example, Garry Marshall uses friends and family members in every film he directs; usually finding a place to jump in the crowd along the way.