You get it in the cave of origin aka ice cave . You can't use it until you beat the sootopolis gym leader.
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HM 9 is Waterfall.
Its in the cave in sotopolis city i think it was called the cave of orgins
1. Beat all the Gym leaders 2. Teach a Water-type the HM Waterfall 3. Use Waterfall at the waterfall by Victory Road
find the hm waterfall go up the waterfall where the elite four is go through the cave face the elite four,you beat the game and just capture more Pokemon
Moon Stone can be found in Meteor Falls, you must get HM07 (Waterfall) before you can get it. :)