A Double Cash Card, or more commonly known as a "DCC", is an item that you can receive from the daily bonus. Like any other daily bonus gift this is gifted randomly.
A DCC is used to double the number of cash you purchase with real money. So, for example, if you buy 5,000 A-O Cash, you will receive 10,000.
A DCC makes your "buy" icon turn red and has a 2x in front of "buy", to indicate that you can purchase twice the amount for your initial intended purchase value.
No, you can not purchase a DCC.