Find out who the site manager is, and talk to him. He will be in charge of the hiring and might have an inside track on where to go to get your card as inexpensively as possible. If he ends up liking you, it could very well turn into a situation where he just says "Consider yourself hired. Go to X to get your guard card, and when you have it, I'll put you on the schedule." Since you aren't talking about a gun card, I'm assuming it's an unarmed position?
Some advice if you get the job: Some posts are dead-ends in that you can never really climb the ladder; you'll just always be a guard. In others, there is plenty of room to move up to supervisor/Sgt./Lt. positions, which is a good thing. Remember that security guard positiosn get really boring, really fast, and when the mind wanders, it is tempting to do stuff that goes against the rules. Don't do it. Keep your nose clean and try to move up, because you can make pretty good money. Even if this ends up being a situation where you can't advance, you can get some good experience and go to a better company down the road. If you can get on with a company that has government contracts and pays prevailing wage, you can make REALLY good money.