What's your sewing level? Newbie? Comfortable with patterns? Comfortable with adhoc-ing it?
If you're not comfortable with as much "making", then yes, go ahead and find existing clothing articles that you can use for the costume: close-fitting shirt, leggings, etc. If you're comfortable with some sewing, but haven't done a costume before, then I recommend what I did when I was first starting. Find an existing pattern that looks as much as possible like your goal, and then you can tweak where you need to. They have patterns for gloves as well. The boots might be tricky. I still struggle with having to create my own costume footwear. If you can find something existing, GET IT. Otherwise, get a base shoe (or slipper) and then build around it (cover it with fabric, fit fabric over your leg, etc) and use some interfacing to make it stiff looking.
Dungeons & Dragons : Sheila the Thief
Adventure Time: Flame Princess
Kaichou wa Maid-sama: Misaki (Maid Latte uniform)
Chihayafuru : Chihaya (red/blue tournament outfit)
Avatar: Ty Lee