Six months after their introduction in Reno and Sparks, Ca$h Mobs are spreading elsewhere in northern Nevada.The largely informal initiative, which calls for consumers to spend $10 or $20 at a locally owned business on a designated day, has brought a strong shot of sales to a half dozen companies in Reno this year.Retailers say it's hard to track the ongoing benefit after the mob departs, although they're certain that Ca$h Mob is a gift that keeps on giving.Spearheaded by Randy Pease and Steve Funk of PROBIS Ltd. of Reno, the initiative each month identifies a locally owned retailer that …