So the only thing we bought through Parker was the skirt. They are sized based on waist measurement. We bought 2 -- on "today's" size and one one size up. The skirt is good for 7 years, uniform-wise, so it's worth buying. When you go for orientation, PTA will have a uniform sale -- I'd go there first thing (get there slightly early, it was in the small gym last year) if you want to try to buy a 2nd hand one. Skirts are by far the most popular uniform item. The girls wear them with bike shorts or athletic shorts underneath, so they are definitely a multi-wear-before-wash. I think you could get away with just 1.
Hair ornaments need to be black or blue or blend with your hair. Parker has some cute bows and headbands (not sure if I've ever seen anyone wearing them, though). Cut hair things in black are popular (kitty cat headbands, "Friday" bobby pins), but J. for the most part just wears a ponytail or silver clips or a headband.