Pema, I'm 5'0" and about 107-109 lbs. About 80% of my clothing is from labels specially designed for petites, such as Ann Taylor Loft, Banana Republic, Ann Taylor, and the petite section at Macy's. Jcrew also has a petite line, but only available online and I'm skeptical about the consistency of their fit, so I rarely order from them.
If you are shorter and/or thinner than me, those stores should be able to accomodate you because they go all the way down to 00's.
The only exceptions are:
1) Jeans. I need a "normal" rise for jeans, not a petite rise. So I do find I have much better luck shopping non-petite jeans brands.. and then getting them hemmed. I have the best luck with Citizens, Earnest Sewn, and Jbrand. I.e., premium designer jeans. They are worth the $.
2) Sweet Pea tops. I can confidently order an XS in this brand and it fits.
3) H&M. A lot of their line is aimed more at junior-type sizing, IMO. So I can find things there.
4) Occasionally I find an XS or 0 in a regular line that fits okay off-the-rack, and will be perfect with an alteration or two. Honestly though, it is usually way too much trouble to try on a zillion things that don't fit and I can get discouraged easily. BCBG label tends to work okay. There are a few others but I'd have to go read through my closet.
It also helps to know where you are shortest. I'm pretty much short all over - my vertical proportions are pretty proportionate. But for example my DD has very long legs (needs long-length jeans), and yet she is even more vertically petite on top than I am. She has a real challenge finding tops, even in the petite section. They're always too long on her from shoulder to bust line, and then not long enough from bust line to hip (because her torso is normal length).
