Ami won her mystical Transformation Pen by playing the Sailor V game. Luna gave Rei and Makoto their transformation pens, and Artemis was the one who gave Minako her pen, all pens look identical except Minako's, as hers was a different style more like a pencil with a cap and a star on the top.
Each pen has different colour similar to the Sailor Guardian who is using it. It also has a planetary symbol on top.
Pens are not just for transformation they contain sacred powers each Guardians very life force.