I don't know too much about these rifles but I just took a 1916 manufactured St. Etienne one out of the grease last Saturday. P&S Militaria in
Canada
has some repro clips for 20$ each, I am sure if you contacted him he would be able to ship some to the US (but it likely would cost a fair bit) otherwise I would just search on Ebay to find some.
Keep in mind the hardest thing to find for these rifles is the ammo, 8mm Lebel hasn't been made in 60 years other than what Privi Partizan made a couple years back (and the Privi Partizan ammo doesn't have the 'safety groove' around it to keep the pointy bullets off the primer if you use it in a Lebel). You can still find the Privi Partizan Brass but for the most part it is a reload only caliber. If you do find surplus ammo (still shows up from time to time) it tends to be about a 50/50 if it shoots or not as it is so old at this point (WWII manufacture or earlier).
Personally I think they are cool rifles, but I won't be shooting mine at least for a couple months until I get fully set up reloading for it. Other than the ammo and clip issue the main things to check for is the bore condition and the other standard things to check when looking at a rifle. Remember if it is a rejected American manufacture all the damage will have been done by civilian owners, not in a warzone.