Just don't try to farm creatures that are TOO low-level if you want crystals. I guarantee, if you fight something that conns as Too Weak To Be Worthwhile, it will not drop a crystal. No exp = no crystal.

At low levels, you can farm Yagudo in Giddeus for wind crystals, but by the time you hit levels 15-20, the Tier 1 Yagudo won't drop crystals anymore, and the Tier 2s are a major pain in the hoo-has, so it's time to switch to Crawlers in East Sarutabaruta. They'll be TWTBW, so they won't drop crystals, but everything else they drop is worth at least 200 gil a shot. Silk Threads (very very VERY rare drop) are worth 900. Problem is, nothing they drop except silk is stackable, so you spend a lot of time running to the AH or quest NPCs to unload your inventory