Our first contributor (remember, you can suggest or share content here) brought up ASOS - a great site if you're looking for a mainstream look while still sporting a rather tight skirt. While their offerings change constantly, they've often featured rather slim skirts and dresses - particularly designs that don't feature a slit, therefore visibly impeding the stride while not overtly looking extreme. Here are a couple of current examples:
What is particularly great about the ASOS site is that they feature runway videos of all their products, so you can actually see how the garments look on a walking model. I highly recommend checking it out - you'll see why I included the pieces above. You can find several similar skirts and dresses by simply doing a search for "maxi." Also, I want to point out that the link here are to the US site, but ASOS has several other regional variations as well.