What region are you in? That matters because these calendars are usually put out by specific fire departments or local charities that support the emergency services. They're actually a real thing, not a joke - it's a way to raise money for the department's needs, equipment purchases, or helping victims. The calendars can feature firefighters in full gear and uniform, or in a more casual style - depends on who's organizing it.
Look through the official channels of your city's fire department or through the social media pages of organizations that support emergency services. You can also try searching through charities that work with fire departments. The key here is not to end up with some sketchy "calendar" pretending to be charitable while the money goes who knows where.
One thing nobody mentioned: if you found an ad online, make sure it's actually coming from an official source. Scammers often use the charity angle to sell fakes. Reaching out directly to the fire department or checking the official foundation website - that's your safest bet.