I work in an extremely casual office but I dress up most of the time, and often I wear pencil skirts with nice tops tucked into them and a belt to cinch my waist. In the winter, I wear tights and heels or boots and a cardigan or blazer with it, in the summer I ditch the cardigan and wear strappy sandals or ballet flats.
I second Loft (that’s where I get most of my clothes), as well as Banana Republic or Ann Taylor. I’m on a budget so I go to good quality secondhand stores a lot, and I’ve found a few very nice tops from J Crew there as well, so you may want to check that out. I think Modcloth is for a more casual office or for activities outside of work. I have gotten several dresses there, though, that I wear to work regularly that are solids and pretty conservative while still being stylish.
I’ve never had to worry about a skirt going past my knees, though. I’m very petite and if I did that it would look horrible, so I’m pretty sure all my skirts are above the knee or just at the knee.
Why, I am glad you asked! I wear walking-friendly men’s loafers, with a wider front so that my toes don’t get squeezed. I walk to work in my walking-friendly-but-bad-climate east coast city so it is important that my office shoes are comfortable.