Alta Loma is part of Rancho Cucamonga, according to the new Southern California Frequency Directory (www.CaliforniaScanner.com).
Rancho Cucamonga is patrolled by San Bernardino County Sheriff, and they use talkgroup 304 on System 6 for most traffic around there. It is obviously all listed in the SCFD book (as well as fire station resources, etc.), but much of what you need is also listed on this website at: