Job interview attire for very petite women?

Even though I am in my mid 20s, I have a small body of an 15-year-old. In fact, I fit my 15-year-old niece's clothes nicely. So I have a very difficult time finding a blazer that looks professional and fits well. In fact, I've been trying for months. Even the blazers/suits in the petite section of Ann Taylor, The Limited, JCrew and Banana look extremely big on me. They made me look like I'm a little kid wearing my father's blazer. So my question is: is it necessary to wear a blazer/suit to a professional job interview. What are some other options besides suits? Would a nice pencil skirt and a cardigan work? Or pencil skirt with a sweater over a collared shirt? I'm contemplating wearing one of these two combos. Thanks.