Hey -- you are a 1/4 inch taller than me! But also a lot trimmer, so I totally understand how your frustration could be greater than my own. I take a 2p-4p in BR & AT... if you are in 0p's or 00p's the challenge is even more difficult.
I shop regularly at all the same places as you do. I can add a few places you might want to check out. I am going down the directory listing of my local mall here... thinking about the places my daughters (before they grew) and I had luck at...
Forever 21 -- kind of like an H&M (fits are similar), but cheaper (comparable in price & quality to Target), and more junior styles (like LOFT vs AT). You have to dig and be choosy, but I have had some surprising success here.
Delia's -- my kids loved to shop here when in high school. They've grown 4 inches and now tower over me. But DD#2 still often finds her jeans here. They fit me too, I just find the rise too low. Most of the rest of the store is tee shirts/casual wear. But the jeans are worth checking out.
344 - more upscale, more $. Not sure they have an online presence?
Bebe - gotta be choosy but I can routinely find stuff that fits here, pretty easily.
Club Monaco - worth a shot. I have to size up to accommodate my figure, then the vertical proportions are all off. You might not have to size up -- though you may still have to hem here and there.
Gap & ON Kids - for jean jackets, and maybe even jeans. I am a size 12 kids in those brands, when my weight is lower. Might also try for button downs.
Abercrombie - fits in shoulders but not length and sleeves. However, sometimes you can find something that is easily altered (i.e., shoulders and armholes and waist fit perfectly -- it's just the sleeve that is freakishly 6 inches past the tip of my longest finger??)
INC brand at Macy's - available in petites. Have you tried this brand yet?
Madewell - for quality basics. Not petites, but if you are slim petite, you may find you have some luck. Try their jeans. Their jeans fit surprisingly good.
Talbot's petites - worth a shot, especially if you are a racy rectangle with slim waist. And like their color palette.
Urban outfitters - go for the smallest size.
Wet Seal - don't laugh. Good for basics, like cami's.
Love Culture - kind of like a F21, with bad lighting.