I'm sorry this is late but I will post in case you deferred your trip and for others. I was actually testing this route in October for my Personal Guide: Rose Gardens of Paris. See www.gaias-gift.com for more information on the Guides.

Since I book people into a known left-bank hotel along the Seine, the Guide offers very detailed directions, even how to walk from the hotel to the bus stop. However, the short version is that I advise people to take bus 47 from that left bank location, direction Fort du Kremlin - Bicetre, to Port d'Italie (NOT Place d'Italie) and then take bus 184 in the direction Les Blondeaux. Exit Sous Prefecture-Eglise. The park entrance is across the street from the bus stop and slightly up a hill, to the right of the historic church.

It's a lot of fun to watch the bus driver manuever the big bus on the small streets of the old suburbs. In late October it was rainy and cold and I've visited the garden before in May and June when it's at its best, so I checked out everything to make sure I had good directions and then turned around, went back to the bus stop and caught the same bus on the return trip. The bus driver made a funny surprised face at me. Tried to explain but not sure if I was coherent in French.