dupont makes something called Tyvek that is a non-tearable washable 'paper' like product. We sell a line of baby books that is made from it at the bookstore. I am not sure where you are but here is a link to the dupont product page that should be a starting point to finding a bulk source.
You can get tyvek in the form of envelopes at staples. I just cut those up to make slips.
As far as bulk goes, it is used in construction, so could likely be purchased by the roll at a hardware store. However, that may likely have printing on one side.