You can buy any box that you think will fit in there, trial and error. Kleenex doesn't market a box specifically for the car. They do have products called "juniors" or "softbox" that will probably fit in the box in the car.
Look for the 1" high flat square kleenex box in the wal-mart checkout line or grocery store line. works fine in my 1997.
Most camping stores and travel stores have exact size boxes that fit in the holders. Also you might check with marine stores as well. Not hard to find; even some of our local supermarkets carries them.
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