World Cup flags are now available for free in Restaurant City via the generosity of the Gourmet King. He's the guy in the purple suit and white wig that randomly appears in the game. You can usually spot him strolling the sidewalks outside your restaurant or picnicking by your vegetable garden. Clicking on him will give you the choice for one of three freebies to share with your friends on your Facebook Wall.
Right now, fifteen premium priced flags are offered in the game. Countries represented are USA, Ivory Coast, Germany, Italy, Argentina, Spain, Denmark, Netherlands, Portugal, Australia, Mexico, Brazil, Chile, France, and England (which, I should note, is of the St. George's Cross and NOT the Union Jack). These flags normally cost 4 Playfish Cash (where the lowest premium cash package is $5 USD for 25 PFC). You can find all of the flags at the in-game store under "Wall Decorations".