The way i found work as an extra was through a CASTING AGENCY. They are contracted by productions to find and supply extras.
The casting agency I worked with was onlocation casting. Don't know if they are "the best" I just knew that they had a need for extras in my location. There are probably lots of other agencies. DO YOUR OWN RESEARCH.
It is my opinion it shouldn't cost you anything to be an extra. Don't pay for listings, professional head shots etc. It has never cost me anything to be work on several productions. AGAIN DO YOUR OWN RESEARCH.
For me being an extra is all about stereotypes. They have a look they are after. College students, stay at home mom, grandmother, biker, camper, whatever. The agency will be trying to get extras that fit the productions needs that's all. So if you are a good fit you're in. If you are not they just won't use you. It isn't personal. You either are or aren't the TYPE needed.