Hi Rusty Neon!
Marina has already mentioned the Gibert bunch of bookstores - they will be all around you when you get off at the St. Michel stop, just on the other side of river from the Ile de la Cité (Notre Dame). If you are walking from Notre Dame, cross the bridge on the left hand side, and cross the main street. Turn left on that street and go about 50m (I think). The Gibert metaphysical bookstore has a small window and small door, but you won't miss it if you read the signs on the way up.
If you get off at the St. Michel stop, turn back towards the river and go up the main street, before it crosses onto the island. Turn right, and go down about 50m.
The tarot decks are in a big glass case next to counter, there should be about a hundred of them in there. There are also some (last time I was there) stacked on the same case, but kind of behind the counter. You just have to stick your neck in and study the boxes, and ask to look if you find something you want to see. It is a very crammed store, but is very well worth spending an hour or two in. I'm sure you'll find something you like. I found an Il Meneghello Tarot de Besançon, and a few others I can't remember now. They have used as well as new books.
The other good tarot bookstore I have found is called "Librairie de l'Inconnu". This is in the Montparnasse area. They have scores, if not hundreds, and it is a very beautiful bookstore as well. It is on rue du Cherche-Midi, no. 84. There are multiple metro stops you could choose, but none are particularly close. Fortunately, it's open every day but Sunday, from 10 to 7 (no lunch closure).
Issy - Spoonbender mentioned the French Museum of Playing Cards - you *have* to go there to see the permanent exhibition. Take line 12 and get off at the Mairie d'Issy stop (it's all on the directions on the site - go to the first diamond lower on the page (Infos pratiques), and you'll see their hours, fees etc.).
When you get off at the stop, going up to street level you will find yourself in a large plaza, with cafés and shops. Go up to the busy main street, and go right (opposite direction to Paris). You will have to walk a few hundred metres along that street, crossing a few other small streets, to get to Auguste Gervais. There will be a big sign on one of the buildings, high up, as you go, so you shouldn't get lost (it has the Museum Logo on it, with the broken squares/playing card symbol). When you get to Auguste Gervais, the north corner will have a café - cross there. Go up the hill a hundred metres or so - the Museum will be obvious. The main entrance is sliding glass doors, and it opens to a long glass-lined chamber with displays, with a round desk at the end. That desk is also the shop, with the items for sale displayed under the glass. They have a large selection of cards and game items, as well as tarot cards (around at the back of the desk if they haven't changed it). The permanent exhibit is downstairs, and temporary exhibits are upstairs. Both are from a door to the left of the counter.
It is an essential part of any Tarot Tour of Paris, so don't miss it.
The Bibliothèque nationale might be tricky. I haven't seen any of the collection personally, and I understand it is not on display. You will have to make an appointment, or plead once you get there (or make an appointment for later). The site is the site Richelieu, downtown, not the Mitterrand site (that's the big one with the four giant buildings at each corner).
For the Richelieu site, the best stop is La Bourse (multiple lines). Go south on rue Vivienne. Don't go into the main entrance on r. Vivienne (unless you want to go look), but go to the bottom of the street and turn left. Go around into the other entrance, which is where the Cabinet des Estampes is located. Be sure to check your hours, and days. I think the Cabinet des Estampes is pretty odd in that respect.
I have heard that Mlle. Lenormand's grave is in Père Lachaise, but I haven't managed to find it on two trips... still worth looking for anyway, while you're strolling around the cemetery.
I'm sure there are other treasures in Paris that I haven't found (or remembered), but if I remember them I'll post them.
Have a wonderful trip!
Ross