The rates are going to be higher at that time of year, but making the cheapest cancelable reservation is good advice. (I took a picture of the National lot during the holidays this last December because it was so funny - TWO CARS in the entire lot. Not very funny if you need a car, however.)

After making a reservation (and checking with all your usual sources repeatedly), I would just keep trying Priceline repeatedly for the next several months. I generally start at half the lowest cancelable rate I have reserved and then bid up to a daily rate that will give me about the same total price as my reservation (if necessary). You still might not do much better than $25 at day, but you've got many months to try. I just wouldn't bid on the high side until many months down the road.

My cousin is here currently and got a van because we're working on our condos and hauling stuff around. However, for the second half of her trip, she hasn't been able to find even an economy car for less than $50 a day. Such is the price for waiting, but it just goes to show you that right now all the companies are charging high rates for just about any time.