What is the widest range 9 speed cassette?
If you are looking to go faster on the flats you can find multiple cassette models with a range of 11-34. If you are looking for more "granny" on the hills, 12-36 is the best you are going to find out of the box without swapping out the low ring. (Sheldon Brown)

That said, on most 9-speed cassettes I have seen, the smallest ring or two are not riveted to the body, so you could easily swap the 12 out for an 11 to get that much more range.