Many game stores and comic stores sell used Pokmon cards and they might have gold cards available
With that said, for me this list begins with Aethersquall AncientAethersquall Ancient, which while expensive, is the threat that I have seen win energy decks the game time and time again
If thats worth three mana, then go for it
Its often worth giving your opponent the token to ensure your spell resolves or their winning piece doesnt
Because all the cards in a color can be cast using that same type of land, they are vastly more likely to appear in the same decks, leading to consistent, recognizable play styles across decks which play that color
Familiarise with Your Deck : Play numerous games with your deck to gain familiarity with card interactions