I actually don't ever find the need to shop petites, and I'm 5'1" (don't know my weight at the moment, but usually take XS/S and anywhere from a 0 to a 4). I don't really have problems with shoulders or sleeves or anything like that; my fit problems are generally restricted to too long pants and too baggy waists, and I've figured out enough workarounds for these that I don't tailor too much. It may be that my style skews a bit younger/trendier than many, and that I often have luck in more juniors-type stores?
I also live in Canada, so we don't have j crew or LOFT. I tend to shop at quite a few stores I believe are only in Canada, but beyond that, Forever 21 sometimes works, although its very hit and miss, and I recently picked up some stuff at Zara, but in the girls section. H&M is actually pretty good, as things tend to run small. Although these are basically all fast fashion/trendy retailers, you might want to try them anyways, because there's so much stuff in these stores that you can sometimes find some really great and classic-sort of pieces. I pretty much just shop everywhere and try on a ton of stuff. I've learned some tricks for more casual clothing-- elastic waisted skirts are great for fitting at the waist, and jersey dresses are awesome for me because I'm really small on top but bigger on the bottom, and I don't have as many issues with fit. Jersey also stretches, so things in jersey are almost never too big for me! Skinny jeans or jeggings are actually a really good option because you can just scrunch the bottoms or fold and tuck under and its stays without any actual tailoring.
