Hi, I'm thinking of getting crystals to aid meditation practice, specifically to improve mental clarity.

I'm thinking of getting it off websites, and I am aware there are different kinds of crystals. Should I get quartz or already polished crystals? Any difference?

you can use any crystals or stones for meditation. It depends on what you want to meditate on. I have had one of the best meditations where we sat or layed down on the floor and put 6  clear quartz points in a triangle formation. two quartz point to point on the left side near bottom if sitting, two on right side and the last two out in front of you. You have to make sure they pointed in a triangle formation.
I go to a group guided meditation where we are offered stones to choose from or the stones choose you, sometimes I pick rough stones but most if holding the stone/crystal then polished so it is smooth to hold and doesn't dig into hands.