How to get an agent if you're a background artist
Whatever you do, don't give your day/evening job! Not just yet anyway, especially if your employer's flexible about your working hours or you can pick and choose your hours because the majority background agencies will only have their books open once a year and if you miss it, you'll have to wait and you might only work once a month to begin with.
All the reputable well known Background artist's agencies are listed in "Contacts", published once a year by Spotlight" and are also on the internet. A word of warning about "internet agencies" they usually ask you to pay a fee and the quality of the jobs, if you actually get any through web agencies is debatable and then there's the issue of being paid, you probably won't.
Spend some time researching each agency, then give your chosen agencies a quick call and ask when their books will next be open and how to apply to be registered with them. Keep a note in your diary and if necessary call back nearer the registration time, so that you'll have plenty of time to fill out forms etc and to arrange a registration date if they want you to go and see them and have pictures taken there and then.
It's also a good idea to be registered with a more than agency if you want to build up your work load, and in the majority of cases, it can take 12 weeks to be paid and it's always after the agency has received payment from the production company, taken out their percentage in commission and then they pay the artists - another reason why not to give your day job.
Once you're up and running, join BECTU and maybe Equity who's membership will usually give you automatic public liability insurance while at work and help you with any contract and pay disputes.
Oh, and arrive at least fifteen minutes before your call time when you got booked for a job, then you'll have more of a chance of being re-booked sooner until eventually you could be in a position where you may have to turn down work because you're already booked and working.