Lots of luggage and outdoor equipment use Nylon Cordura of 1000 denier or higher, which often already has a waterproof coating. A quick Google shows lots of UK companies that sell it by the meter (£10-15) or will send you a sample pack of available colours for a few quid. I don't know how big the samples are, but you could always drop them an email.
Don't get distracted by some super heavyweight indestructible fabric though as the tougher they are, the harder they are to work with.