You could try
http://www.waterproof-paper.com/ they have a bunch of different waterproof bags, I have a few different sizes and have been whitewater rafting, skiing, hiking, kayaking and even surfing with them all and have never ever had a leak. I would also suggest getting a packet of the Silica Gel Packs, I think there were 5 small packets in the package I bought. We use them here because of the high humidity (80% - 95% all year round). You don't want to start with moisture in you bag before you get near the water.
I can't vouch for the above site, I didn't buy my bags online, I picked them up locally in Canada at a place called Wholesale Sports, I have seen them at MEC (Mountain Equipment Coop) as well. I don't think they were a lot, I'm rather cheap
I would also second Harry's suggestion of Map Cases, But you might find the sizes too big?
I've attached pics of the few sizes I thought you might be interested in.
Cheers