I presume for some price, a skilled witch or wizard could make tons of this stuff and Harry (or Dumbledore or Voldemort or Snape or Belltrix etc. etc.) could just buy some.
Firstly, Felix Felicis is dangerous in excessive quantities. So it would be self-defeating to simply overdose. And secondly, it makes someone lucky. And whilst this is just a guess, it is probably useless when it comes up against true skill (in the battle at the end of ‘??? ?, the Death Eaters were not facing them in a one on one duel, it was more a general melee). So I can’t see Voldermort beating Dumbledore in a duel, for example, just because of Felix.