I'm 5'1" and 105 lbs. Finding clothes can be quite tricky for us small gals.
For nicer, more business oriented clothes, Ann Taylor is THE place to go. Their sizes go down to 00. That's where I started buying my slacks after Petite Sophisticate closed its doors.
You can often find gems at places like Forever 21, Charlotte Russe, and Wet Seal, but often times it's real hit-or-miss. Try it on before you buy it. If you have an H&M in your area, hit that up because when I did live near an H&M, I found some really great stuff that fit me.
I find that Old Navy's sizes run huuuuuuuuuuge. It wasn't that way a few years ago, sadly, because sometimes you can find cute stuff. If you're under a size 4 or 6 (which is their size 0), you're probably not going to find much. Same with the Limited.