I suspect the original poster bikepacks below sea level!
I concur with above that Supplex is a good material choice... it is has the feel cotton, but is made of nylon (quick drying). It is not 'glossy' like ripstop and is not coated with waterproofing. Supplex is breathable.
A quick search found this place in the UK:
http://www.profabrics.co.uk/, they seem to have everything except Supplex. Might want to contact them for a source.
... Found this
http://www.pennineoutdoor.co.uk/search.asp?query=supplex, it would work, but the stuff I'm thinking of doesn't have lycra in it.