I bought a skirt suit from Ann Taylor. I just went there and said, "I have an interview. Dress me." I didn't go during a sale and I didn't shop around because I needed something fast and of great quality. It got expensive real fast, but these days Ann Taylor has some great sales and if you're lucky you can get some matching pieces deeply discounted. I just saw my suit jacket for about 75% off. It was the petite size, but I was still kicking myself. They also have some great shoes on sale. It's so hard to find fashionable yet conservative shoes these days! (with a sensible heel!)
It really doesn't matter whether you go with pants or skirt. I've interviewed at 3 places, and I only saw one other girl with a skirt. (actually, come to think of it, maybe none!) My dad insisted that I go with a skirt suit because it's considered more conservative for women, it happened to be my choice too, so it all worked out. Just go with what you're confortable in. When I was shopping, I was between a black skirt suit and a grey, herringbone one. The black was just your typical female powersuit, and the sales rep was really pushing it. My boyfriend was also pushing it, but I realized that his thinking was that if I go with black, I'll look like everyone else and blend in. He thought that would make me feel comfortable. But really, I feel more comfortable when I stand out, and the grey suit was more flattering, more unique, and more feminine, so I went with the grey.
good luck!