Well I can only speak for the Indian restaurant scene in Paris, which leave a lot to be desired. There is at least one Indian restaurant in every quartier, but they are all identical, with the same overpriced menu of inauthentic dishes barely recognisable as vaguely Punjabi in origin. The most popular dish seems to be "cheese naan". When I first heard of this, I imagined a naan stuffed with paneer, which sounded bizarre but appetising. How wrong I was. Paneer is not available here and completely unknown, so there is no need for chefs to make their own when they can so successfully pass off generic French cream cheese - yes, cream cheese - as "fromage indien" (sometimes even cheekily referred to as "fromage indien maison"). Hence mattar paneer becomes a sort of creamy pea stew.
However, just yesterday I was shopping in the gourmet hall of the Galeries Lafayette when I noticed an enticing array of familiar and startlingly tempting dishes in their traiteur section, including samosas, pakoras, tandoori chicken, biryani and mattar paneer with what looked like real paneer! No way! I instantly enquired as to the caterer of these delicacies and found that they had originated from a restaurant called Le Palais d'Archana, on 110 boulevard Charonne, in the 20th. However, whether the apparent quality of the food is indicative of the standard of the restaurant or the result of a special effort to impress the discerning tastes of the Galeries Lafayette clientele, I do not know.
I have also recently discovered the South Indian/Sri Lankan quarter, which occupies the stretch between Gare du Nord and La Chapelle. This area is home to quite a few interesting looking South Indian and Sri Lankan restaurants which I have not tried as yet (don't know anyone else who likes Indian food and don't like going out to eat alone) but which, judging from the lower prices and the fact that all the street signs and shop names in the area are displayed in Sanskrit only, promise to be much more authentic, at the very least.
As for ingredients and so forth, I tend to frequent the traditional and original heart of the Indian scene in Paris, which is the passage Brady, in the 10th arrondissement. One of the stores, Velan, even has
its own website now. It's a pretty good place to get all the essentials, and even to stock up on such things as walnuts and dried fruit which are much cheaper there than in generic French grocery stores.