I don't have a guaranteed source or any real info with which to help you. However, here is the contact information for a cool and reputable photog supply house here in Indy. I purchased all of my B&W darkroom gear that I use for school from them. I have not spent a lot of time in their store and have never looked for paper in those sizes. I also cannot find much info on their website. Your best bet would be a phone call.
Roberts Imaging
Downtown Store
255 S. Meridian
Indianapolis, IN 46225
317-636-5544
1-800-726-5544
Roberts: Photo • Video • Electronics • Imaging • Since 1957