"Silnylon" seems to be a slightly broader term than the silicone elastomer coated high tenacity ripstops used by e,g, TNF, TN, Hilleberg etc. There are some PU coated nylons with "a silicone treatment" which I doubt would be as good (PU coats will heat-age the fabric far more than a SE coating).
So I wouldn't stop asking questions at "is it silnylon?"
Point North do a silocone elastomer coated ripstop fabric (the first item on this page):
These guys also do Silnylon, but appear to be out of stock They're in Finland, but very helpful, speak English, and it's a lot more convenient than the USA!
Zombie thread alert: six years old...
Tyvek is used for building purposes, so should be available form any builder's merchant. Unfortunately, probably only in huge rolls... Maybe worth trying to scrounge some from a building site skip...
Link to BPL article on waterproof materials
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