They're not a common drop by enemies, although you'll find a couple here and there in treasure orbs or as a drop. The Survival Catalogue will only increase the drop rate by so much depending on your characters. Also, if you score no stars on a battle, there's a higher chance you'll get some.
So pretty much... you don't get rewarded very well for being a good player - play badly, and you'll get the treasure. =/
Later on in the game, you gain access to Eden Pharmaceuticals (in the last chapter... helpful), where you can buy them (but they're really freaking expensive if you're low on Gil - it's not worth it). I believe Ethersol can only be gotten through dismantle, but I'm not sure what you have to dismantle to get it. Anywho, you can buy them for:
~ Fortisol - 12,000Gil.
~ Aegisol - 12,000Gil.
~ Deceptisol - 30,000Gil.
As helpful as they are, you shouldn't rely on them too early in the game. Enemies are fairly simple at the start anyway, and you'll get more than you know what to do with early on.
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