You will find in every mission and in Paris yellow Supply crates.
You know, these yellow boxes w/ grey or white lid.
When you loot them you will get money and xp.
But don't worry, some of them are refilled, when you leave the district and come back later again, so you can loot the same crate more than one time.
@Doombot98275: i Think you can come back after one day.
I played the first mission again and again and after one day the supply crates are refilled.
Happy hunting.
Greets from Hamburg, Germany