Parts Unlimited has a couple of options. The $50 one is pretty good. You can pay more for fancier linings and stuff. It's probably worth it to pay as much as you can afford so it will be breathable as well as waterproof. Here in the South, a plain tarp will rot your bike from underneath because the moisture gets trapped. Waterproof works both ways, I'm afraid.
Would they permit a small tent, or tent-like structure. A couple of companies make those, too.