Emhar and Tauro indeed have WWI armor. Emhar has a growing range of 1/35 and 1/72 lits, plus 1/72 figures to go along. Tauro, if you can still find the kit, had a A7V in 1/35. Airfix has a 1/76 Mark I Male in its range, old but still very decent (it's a MkII in fact, but that's not a problem). Matchbox had a Renault FT in a box with the Char B1bis (with WWII markings but that can easily be changed to a WWI vehicle). RPM have been releasing countless FT, many of which are WWI.
As far as mainstream model manufacturer, that's about it, I'm afraid. However, the market is 'awash' with releases in resin, metal or mixed media, from companies such as Retromodels (1/72), SHQ, Scale Model Accessories (1/76) to name but a few.