Sleeves are generally a good idea for keeping your cards in good condition so they don't get scratches, edgewear and worn down with shuffling.
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Originally Posted by pmarckoue
If you plan on using those sleeves for tournaments, I believe you are not allowed to use any holo or shiny sleeves, and only use one color per deck.
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They can't have unique patterns, colors, creases, dirt, different sizes or brands or varying conditions of play(new sleeves clearly stand out from played sleeves). These things are considered "marking" since it allows you to distinguish 1 or more cards from the others.
Tournaments don't require you to use sleeves, but it's recommended since it's much more likely someone will call you out for marked cards or a deck check if they legitimately feel you're cheating or they feel like being a dick.
Sleeves can be any color, translucent, whatever. And all official Pokemon sleeves can be used.
Depending on where you buy them from, you can get standard-sized 60 sleeves for $5 or 100 for $6. But that's just basic sleeves. Official Pokemon ones tend to get rather expensive.