When you train to be a Rescue Diver, and especially when you reach a professional level, you will be expected to have a pocket mask.
I looked around at some before ordering, beginning at scuba dive shops. I noticed that they aren't so cheap. Neither are the official Red Cross masks.
It didn't take long until I found these pocket masks at Amazon. \$8 each is the lowest price I found, and much lower than the price quoted at dive shops. As long as it has an O2 inlet and covers the mouth / nose, you're good.