I second ebay. especially since you mentioned you would like to give out the number before you leave.
Here is an ebay listing. It has already been overbid though. Seller is located here too.
http://cgi.ebay.com/French-SIM-Card-for-Fr...1QQcmdZViewItem
Phones: Xiaomi Mi Mix 2, Samsung Galaxy A50, ASUS zenfone 3,
Sim cards: AT&T (Contract), 3 UK, Piranha Mobile