When you get to world 2, speedrun to the airship and get Laka from Falhud Castle, you can farm Golden Apples in the Everlasting Tangle forest before the dungeon from the Tiki Mask guys. You are beyond good until then it sounds like.
Viewing post in What are good places to steal shards?
Golden Apples increase all stats (except for HP/MP) by 1, so you don't need to farm/buy other shards anymore. On the other hand, though (and I think that's what you're worried about), it seems there's no way to obtain Miracle Fruits (which increase HP/MP) beyond the limited number found in treasure chests etc., but I could be wrong about that.
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.
And just as i continue to play i find another place to farm shards in osteinchyl if you get the guard puzzle wrong on the lever, you get a fight against a chimera monster that always has a guard shard to steal, the downside of this method is that once the puzzle is done you can't get the monster to spawn again.