The helmets are easy to find but a decent true WW1 repro is nowhere near $40... more like $100 + shipping.
Most repros use British MkII shells (virtually the same thing). The spendy part is the liner and chinstrap. They used the same liner as the British MkI helmet without the rubber 'doughnut'. The leather chinstrap is an integral component of the liner. A repro liner runs $65.
In theory you can find a British MkII helmet for $40 or so, assuming the liner isn't dried out, cracked and falling apart (more than likely at that price). Mock up a leather chinstrap and it would be passable from 10 feet as long as its on your head and no one sees the liner. If you want to be more authentic on closer inspection then you need to spend some money.
www.onlinemilitaria.net (aka What Price Glory) has'm...
http://www.onlinemilitaria.net/shopexd. ... 4212&bc=no
http://www.onlinemilitaria.net/shopexd. ... 4540&bc=no