Even if you don’t have a library of World War One books, there are a number of useful websites that provide information about uniforms in that era. Here are some of the ones I’ve found useful.
The Sutlers Stores produces replica uniforms for museum display and docent use. Note the “grayback” shirt which I mentioned in The Moonlight Mistress.
Reenactor.net has a WWI section. It’s not only useful for the information it provides, but as a gateway to making research contacts, if you should want to know what it’s like to wear the uniforms. I love their Morsels of Authenticity, short articles about small details, like German underwear.
Military Headgear at Wilson History and Research Center.
I continue to recommend Osprey Publishing, particularly the “Men at Arms” series books, which feature detailed drawings of uniforms and equipment for a wide range of armies and time periods.
For more idea-sparking material, you can search on WWI at Old Magazine Articles, if you’re willing to spend a little time reading. For example, this Vanity Fair article on American uniforms for the well-dressed, October 1918. Their home page.