Aaron, if you talk to a speciality paper house (aka "paper merchant" you will find there are several types of synthetics (like Tyvek, Kimdura, and some based on latex) that are generally not sold in less than whole ream quantity--but that also ensures you a waterproof and durable chart.
Any printer can tell you who the local paper suppliers are, or order in one of these specialty products from them. There are also "outdoor papers" sold from some of the inkjet companies, which are also synthetics. Available on the web of course, too.
You may need to fiddle a bit with the ink settings, they all don't absorb ink exactly the same way that paper does--the same way that cheap
bond and photo paper differ.