Cake pictured: A chocolate tart with a raspberries I had lying about stuck on top. Verdict: Delicious.
While the French are relatively famous for their cakes, it's not the easiest country to try and pursue cake-making in. Most people seem more content to pop to the local boulangerie/pâtisserie and pick up whatever may be tickling their fancy. I suppose that this is true across the world, you tend to buy rather than make cakes.
Still, there's a point I'm trying to make. Yeast (not present in that delicious cake pictured, don't worry) is not always straight forward to find. In French, the word translates to levure, and indeed you'll find little packets of something labelled "levure" in most shops. Of course, nothing is so simple. If anything is just labelled "levure" then it's most likely to just be sodium bicarbonate (also known as baking soda/powder). Now it's true that baking soda is useful for making things rise, but the two are rarely, if ever, interchangeable.
The full (and correct) term for baking powder is "levure chimique". If you want actual yeast, then you need "levure de boulanger" (or "levure de bier" will work equally well. It's a slightly different culture mainly used in beer, but I really don't think that there is much difference between yeast cultures). Theoretically, you can (if your French is better than mine) buy this from a boulangerie. Otherwise you can just buy the dried stuff if you go to a larger supermarket and double check the labels.
And don't get me started on the ambiguity associated with crème fraiche...
[I'm uploading this in a slightly different way to the usual due to G+ auto uploading my photos. Hopefully it'll all just work out fine]