Sea-to-Summit makes really nice Sil-nylon drybags. They're lightweight, and have a rolltop waterproof closure because they're designed as kayak/boat bags. Seattle Venture Sports makes vinyl drybags which are cheaper and heavier, but sturdier. Unfortunately they're also prone to damage in very low freezing temperatures.

If you're handy with a sewing machine, you can make a pair of fitted drybag liners for your panniers in one evening. Sil-nylon, side-release buckles, and seam sealant can be found online for reasonably cheap.