I don't think you need one, but if you think so, then do what has been suggested. Buy some 2 or 3 mil LDPE (found at the paint section) and cut to shape. Replace as needed. Or, get some tyvek and do the same. The tyvek will last longer, but not as waterproof. You won't notice the waterproof difference, unless you setup camp IN a lake.

If I am buying a tent, I wouldn't want the manufacturer to make the footprint. They are way overpriced for what they should be.
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