Otherwise, the best solution will be a new a synthetic bag. I wouldn't bother with a down-filled bag that has a "waterproof" shell-they're water-resistant, anyway, add $75 to the cost, and cause other condensation problems by trapping moisture inside. One reliable favorite in a synthetic is The North Face's Cat's Meow ($169, 20 degrees, two pounds, 12 ounces). L.L. Bean's Summit 3D 20-Degree Mummy offers similar specs (although it's a bit heavier) for $139.
A third choice: A bag cover. Outdoor Research makes a very fine sleeping bag cover from Gore Dryloft, the stuff used by many bag makers for their water-resistant shell. It weighs just a pounds, but costs a rather steep $142. I haven't seen any water-resistant bag covers listed at Canada's Mountain Equipment Co-op, but you might check the store to see what they have. Makes sense that there would be something more affordable there.