If I remember correctly: By taking your party into the woods south of Laka's house (not directly south, but beyond the stones by using a Chocobo), you can encounter Carnosaurs, from which Power Shards can be stolen. They're strong enemies at first, but you can just feed all the stolen Power Shards to Vahn until he can one-shot the Carnosaurs, which makes the farming process even smoother. You can also steal Power Shards from Behemoths at this point, which appear on the island west of Mohadmal Village (beyond the stones by using a Chocobo), but it's not worth the effort: Behemoths are too strong this soon into the game, only yield twice as much EXP as Carnosaurs, and without the item drop rate increasing accessories etc. that you don't possess at this point, you probably won't get any X-Potion drops from the Behemoths.
Stealing Guard Shards from Mammoths at Mt. Matoya is definitely a worthwhile endeavor, but their appearance rate is fairly low compared to the other enemies in this area.