CEntral casting, located in Burbank, CA, is the union casting agency for SAG, and Cenex, just for non-union extras, is in the same building, and actually they are right across a lobby from each other on the second floor. I did my 18 months as a Cenex Extra until I got my three SAG vouchers and then joined the union. I did not sign with Central, 'cause my "Background actor" days were over. I also used a casting service named Cameo Casting. They are also in L.A. and have the best extras in the business.
Cameo casting website looks like they are calling Central as one unit, (ext. 1 or ext. 2 on the phone call below) so maybe Cenex merged. It's been 10 years since I did extra work.
See
www.cameocasting.com or
Axium/Rich King Casting:323-993-0186 x301
Background Players:323-790-0138 x17
Bill Dance Casting:818-725-4209
www.billdancecasting.com
Casting Associates:310-364-0233
Central Casting:818-562-2755 x1(SAG) x2(Non-Union)
www.centralcasting.org
Creative Extras:310-203-7860
Headquarters Casting:310-556-2626
www.hollywoodos.com
Landsman/Kaye Casting:818-895-4738
Mountain Ash (Pete Sutton): 818-759-1818
On Location Casting:310-229-5332
Prime Casting:323-962-0377
www.primecasting.com
Sande Alessi a.k.a. The Casting Couch: 818-623-7040
Smith and Webster-Davis Casting:310-364-3521
They are all casting call-in services that can supplly you with background actors. I do not know what they charge for their fees. I'd call CAmeo first, but they were the best fo rme, so I'm biased.
Good luck.
WC