For scenario cards, I use TitanShield premium clear card sleeves. They have a matte finish on one side.

For investigator cards, I use Dragon Shield classic clear sleeves. As these ones are a bit more pricey, the only cards I keep sleeved at all times are weaknesses, and cards that are part of scenarios. I have a pack of 100 sleeves that I will use for whatever investigator cards I am using at the time, and I will unsleeve them when I am finished.