Where to find french dictionary for ispell?

I am editing documents in French with Emacs, ispell comes will English dictionary by default. When I try ispell-change-dictionary, it proposes francais-tex, francais7 and francais in the list. But after choosing anyone, and doing ispell-word, it tells me Error: The file "/usr/lib/aspell/francais" can not be opened for reading. Then I check the folder /usr/lib/aspell, indeed there is no "french" file in it.

So could anyone tell me where to find these french dictionaries and put them in the folder?

PS: I am using ubuntu...