I bought a reproduction helmet plate from
Spikehelmets - About Us
"Spikehelmets" is a newly founded company that provides high quality replicas of military headgear and uniform accessories dating to the late nineteenth and early twentieth century.
In the book "The Dutch Steel Helmet" by K. de Joode I found that the helmet plate was designed by Chris van der Hoef, a dutch artist/designer.
It was not easy to get the plate soldered to the helmet but with some help to job got done.