Many social game players — especially people without access to credit cards — have for years been getting virtual currency for online games via pre-paid cards purchasable in physical locations. Social gaming companies have started doing the same, in a trend that we saw pick up last year, with Playfish, Playdom, Zynga and other developers beginning to cut deals with card distributors and store chains.

And now the largest social gaming company, Zynga, is sharing more details on its efforts here. It now has its own, branded cards for games in more than 12,800 locations in the United States. Cards for virtual world YoVille arrived in 7-Eleven and Target during October and November last year. Best Buy GameStop started carrying the cards at the end of February. And farming simulation FarmVille and role-playing game Mafia Wars arrived in 7-Eleven and Target a week or so ago.

Its card distributor in this case is pre-paid card market leader InComm; it separately accepts payments from PayByCash’s Ultimate Game Card pre-paid card and coin-to-virtual currency kiosk provider Rixty.

In case you’re not familiar, pre-paid cards typically provide a scratch off code, allowing the user to buy a set amount of currency for a game at a store — for $10 or $25, in Zynga’s case — then enter the code online to obtain the currency. Then users can go buy special seeds, power-ups, or whatever other virtual items available in Zynga’s games. You can see various options for paying with cards available in Zynga’s offer walls.

While pre-paid cards are only now being tried out by social gaming companies, they represent a promising new way of making money. We’ll be exploring this topic more at our Inside Social Apps 2010 conference next month with social gaming companies and service providers like InComm.