Good luck with that.............. You're going to need to search FN parts vendors for a complete bolt for your Brevette 1912 - sorta like looking for a hen's tooth, at this remove, 100 years after it was made.
You could always buy an entire riffle, though, on GB with a "buy-it-now" price of $250 -
http://www.gunbroker.com/Auction/Vie...Item=265513187
.