Schipperfabrik is one of those rare WW1-(almost)only vendors that a great selection for every side of the conflict. When you see a considerable Belgian and Hungarian selection, then it's a vendor to check out. Schipperfabrik has them. They also have a site that is easy to navigate, and the prices seem pretty reasonable.
As with any sutler I haven't personally bought from in the past, I can't personally vouch for the quality of the items that Schipperfabrik sells. So, as with any sutler, I would collect some recommendations before ordering. Many group sites do recommend Schipperfabrik as a primary sutler.