Am I the only one finding it hard to get  preemptive strikes?
 
Some enemies its pretty easy. Usually where there is only a single foe.
But when there are 2 or more, I can try over and over and they keep sensing me.
 
Is it possible to buy the items that make you undetectable later on or something?

Impossible? No. Hard? For sure. 
 
I believe it has been mentioned elsewhere later in the game you can buy these items from the vendor. 

Yeah you can purchase the the deception shrouds or whatever they're called, but they're seldom worth the cost -- 30,000gil each. You find plenty during your adventures though; you might even have some already -- hit L1 in the overworld to pull up a little submenu of shrouds you can use during your next fight.

Something that I find helps is if you walk slowly, or sneak. Just tilt the LS slowly and your character will walk like they're sneaking and sometimes they won't see you. But Deceptisol is the best, and as Symphony said you might already have some, just hit LB and it'll be there in a menu.

You should never use deceptisol in the first 10 chapters, you never really need it. Save it for the later chapters, you should have plenty of it then. There are tricks to getting pre-emptives, it's usually worth spending 30 seconds to get them as it can cut 80% of your battle time with some enemies. Most enemies you can lure from their position, then they will run back to it. You can get them as soon as they turn around, or you can even make them get stuck like this.