As a former WW2 reenactor, I appreciate a well stocked vendor. Back in my time, there were hardly any decent reproductions, mainly because originals were relatively easy to come by. Amazingly, "my time" was only about 10 years ago, and now originals sell at a premium. What Price Glory is a great vendor, with much to offer, and one of the best rounded selections of WW2 American and British stuff you can get. Both original and reproductions are to be had here - the repros look great.
What makes this site attractive to me is that it has alot of items, and it's well designed, with a simple way of navigating around. As a note - the shoe/boot selection is tremendous. I wish this place was around when I was doing WW2, I could've used a decent pair of boots! What qualifies the WW1 tag are the US M1904 Marching Shoes, which are a nice snag for the WW1 reenactor.
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 What Price Glory sells. So, as with any sutler, I would collect some recommendations before ordering. Many group sites do recommend WPG as a primary sutler.