Would like to visit a Foie Gras farm near Paris
The closest to Paris that you'll find will be in Haute-Normandie, a region that has a long experience of duck breeding. Thus they had no trouble producing very nice foie gras about twenty years ago. The first producers were in Eure and Seine-Maritime, now there are some in Basse-Normandie as well.
They often season their foie gras with pommeau or a bit of calvados which is a pairing made in heaven.
There are good producers in Pont-l'Evêque http://bit.ly/Ma7hEd , near Livarot http://bit.ly/PtTJdw and in the Eure http://bit.ly/PtTYFk - you can fine some products at the Clos Saint-Marc market in Rouen during the weekends.
The Southwest certainly does not have a monopoly on foie gras. Think of Alsace. And there are actually foie gras producers in Ile-de-France (Essonne, Yvelines).
I have been to a foie gras farm in Blennes between Fontainebleau and sens, Auberge des Trois Canards in the village of Villemaugis. I think it is still considered to be Ile-de-France, but is quite near Nemours.
Many foie gras farms in the Southwest welcome visits, especially during the feeding process, quite possibly to show that the process is labor-intensive but not cruel. Although I have had dined (very well) at the Auberge des Trois Canards, I have never visited the farm (sure heard the quack-quack-quacking. I don't know if it does visits. The best bet is to make a reservation for a meal (the restaurant part is open on the weekend only; don't forget it's a farm, a real one), and ask if it is ok for you to come earlier and stay later to take a look at the farm.