Scale Link produce very nice weapons in 54mm.
The best WWI rifles are from Metal Models. You would have to contact them directly to see if they will sell the weapons separately as they are normally part of their figure kits. They make the Lebel but not the Berthier or MG like Chauchat. They also make the Springfield, Enfield and Mauser. I think the Austrian Mannlicher as well.
I bought a half dozen Lebels from Bruno many years ago but they were not cheap. However, they are best for detail and dimensionality.