There's always Julia Louis-Dreyfus from the Seinfeld days of yore and the "New Adventures of Old Christine" days of now. Julia's been sporting the sexy beehive for years and still looks excellent while doing so. Julia used to be a member of the Saturday Night Live cast as well, so she'd be a fabulous choice to take over in case Tina Fey jumps ship. And she's hot. (That's very important! You'll understand how and why later. When you're older.)
Then there's Gina Gershon of the very steamy and very lesbian-y film Bound. She did a video for the folks over there at Funny or Die where you played Sarah Palin. So she already has experience. AND she's good at kissing other women. (Oh, you know you want that to play a part in this Presidential race some how. Admit it. I have.) She would be another excellent choice.
But if all five of them were hit by a bus, the last resort would be for Saturday Night Live to just spend five or six minutes showing this cornfield in Ohio where a farmer had a likeness of Sarah Palin carved/stomped into the crops. It's a little scary, but it really does look like her. That has to be a marketable talent for the guy who does that sort of thing. I don't imagine he's going to get rich off of it though. There are definitely a very limited number of cornfields available for one to mash in the likenesses of political candidates. I mean, it's nice and all, but we do need corn.