There's a huge selection of masks available nowadays - I'd go for the most compact ( avoid panoramic masks ). Make sure it's well clear of your helmet, as you don't want a bump on the head to dislodge your mask. Try on as many as you can, and go for comfort and a good seal ( hold the mask to your face by fingertips, without the strap on, suck in gently through the nose, and see if the mask sticks to your face ). Faces vary a lot, so keep trying different shaped masks until you get a good seal.
Best source is probably your nearest big divers' shop, and don't be frightened by the prices ! Some of my favourite masks have been cheap ones.