She doesn't sell them until you get to the last area of the game. I know tonberry have a high chance to drop power orbs and the Zaghnol in Oerba 400 drops power essence which you need after power orbs. Forked cat sometimes appears with Zaghnol, which is needed for alot of Chichu builds. If you're in Academia 4XX then you can get silver chocobo by throwing mog into the green orb in the room Hope and Alyssa are in. If you're going to farm for power orbs you should get Collector's Catalogues from chocobo racing. They stack in this game so you should get 2. You don't get a graviton core in archilytte stepppe. There is one in New Bodhum 3 AF, Bresha Ruins 5 AF, Oerba I think in 200 AF, Yasschas Massif I think in 100 AF, Academia the one with all the cieth running around, Sunleth Waterscape 400 AF, and Oerba 400 AF.
As a warrior and as a man I will leave my mark upon the world. - Marguilus