Hi, I don't have specific flash cards for this game. I just use a small white board and dry erase markers to write the addition/ subtraction questions on. If you're interested in some cheap addition/subtraction flashcards try your local Michael's Arts and Crafts store. I went to Michael's this past weekend and there were different sets of add/sub flashcards in their discount bins for $1.00.
I just use regular flash cards and the first to blurt the answer moves on. I sometimes choose only cards with sums 10 or less depending on what time of the year I play the game.