In Ontario there are two major plus sized chains. Both carry swim suits and I was shopping with my sister in law (who wears a size 20 and is probably going to kills me for this) tried to find a one piece suit. Good luck! Neither chain store carries one pieces except for one with large cut outs that may as well be a two piece.
She went online to the torrid website and decided that she would try one of theirs. She had it shipped to a friend in Chicago, who shipped it to her, and it fits beautifully. If it didnt at least she was able to try it on in the privacy of her own house, without other women or that harsh store lighting around.