Pleated skirts aren't too hard to make, and there are a bunch of helpful tutorials online that I used to make my first skirts!
That said, my K-On! skirt is a plain gray, pleated skirt that I found on a children's uniform website
. My Mio wears hers high on her waist and I shortened mine as they're a bit long. If you can fit into one of the larger sizes. I believe I ordered one in size 20. It's a little snug, but it fits.
H&M also usually sells blazers. I think they're commonly found in stores around this time of year.