If youre after any of the single starter cards in particular including Totodile, Cyndaquill, and Chikorita you can buy the individual cards from the whole Partner Illustration Collection right now on TCGplayer
If you want to build a top-tier tournament deck (not at all necessary to beat the game), you will have to play many games to accumulate all the rare cards you want
They're also a popular first pick when cracking packs during an MTG draft
What drives the Luffy build cost up from Buggy: The accessible version of this deck avoids the most expensive Green singles and leans on the Impel Down half during the early game while transitioning to Straw Hat pressure mid-to-late
Sylvan is a single two mana investment that if not immediately dealt with, typically allows you to find whatever you need quickly, whether that be mana, removal, countermagic, or another threat