Tyvek is lightweight and waterproof, but it is not extremely lightweight and waterproof. I would suggest 1.1oz or 1.9 oz silicon impregnated nylon, or silnylon.

For tyvek, try http://www.questoutfitters.com/

For silnylon, try outdoor wilderness fabrics with the link from this site, or you can try quest outfitters as well.
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