Contact your local Board of REALTORS® for an application. Most have special catagories for affiliated members such as appraiseers. There will be an application fee, annual fee, and monthly membership fee.
My board dues are approximately $500 annual fee, $45 a month for MLS, and $5 a month for a lock box key plus a deposit for same; however, I have a full membership. An associate fee is slightly less on the annual dues but the monthly MLS and key fees are the same.