No standalone booster packs exist, so you're limited to ETBs (10 packs, originally $49.99), Special Team Collections (6 packs each), and various other collections
7 Rainbow Energy (Secret Rare Holofoil) Celestial Storm Market Price: $31.30 The Rainbow Energy card debuted all the way back in the Pokemon TCGs fourth-ever set, Team Rocket, where it was not one of the most valuable cards but it certainly was among the most influential, as it provided any energy type while it was in play
Flareon may be at the bottom of that list, but it is still sought after if it has the Master Ball Pattern
Charizard's Shiny design is one that's gone through quite a few changes over the course of the Pokmon franchise, as well, making this card into a visual time capsule
You must also track how each became face down (Magar's ability, cast face down using the morph ability, and so on)