You can buy medium rye flour online here. It's also available at some Wegman's locations and on the King Arthur Baking website. I can't check every grocery store chain, but I did a bit of Googling and it looks like it might also be available at some Weis and Vons locations. There are a lot of rye flours out there, but I've only tested this with King Arthur's, so really do recommend using that if you can.