LOS ALGODONES, Mexico ― The billboards that line the stretch of California’s Interstate 8 headed toward Los Algodones make it clear travelers are not on their way to a typical tourism destination. The signs that beckon Americans as they head west toward Andrade, California’s narrow border crossing, aren’t for resorts or beaches but for dental clinics offering bridges and root canals at half what they cost in the United States. Dental care has become big business here over the last two decades, so much so that American visitors have taken to calling it “Molar City.” An estimated 600 dentists operate...