I expect to spend some time in France, and am wondering the same thing.

From my visits to Paris, I suspect there is nothing specially geared to LC. However, the super-markets are there, of course, and there are plenty of "natural" LC products.

To eat on the streets, I guess one can find snack-size cheese, and nuts, and I think that's it.

There may also be some sugar-free chocolate that uses maltitol as sweetener. They market it for diabetics, but it is ok for LC-ing. But maltitol cannot be consumed in large quantities as it has a notorious "laxative" effect (after certain amount).

Of course, one can order stuff from UK and/or US LC internet shops.