Where do i find the best cards to make a rock deck?
i'd say gem knights (they're mostly rocks) or magnets. gems are one of the most expensive decks out there, requiring multiple copies of UR cards from multiple main boxes. magnets are however quite f2p friendly as none of thier core cards are from boxes
A rock deck? The best possible rock deck at the moment is Rock Stun.
For this, you will need Koa'Ki Meiru's (Sandman, Wall, Rooklord, Prototype, Diamond Core & Core Reinforcement), as well as Block Golem (from the Card Trader) & Ties of the Brethren (Abyss Encounters). Other option is Gigantes (Secrets of the Ancients).
Sets to buy for the Koa'Ki Meiru's: Gaia Genesis & Revolution Beginning.
But take in mind that Rock Stun has issues due to missing some interesting cards that really makes it work, like Fossil Dyna, Koa'Ki Meiru Guardian, Grand Mole & Legendary Jiujitsu Master.