I've found The Children's Place to have the most consistently modest and appropriate clothing for girls. This is a mall chain, so see if a mall in your area has a Children's Place. You can get solid shirts (they sell some "graphic tees" and stuff with a little glitter at the neck or whatever, but none of this junk covered iin characters and crusted with rhinestones and sequins), solid matching trousers, jeans and jean leggings, etc. Not super-cheap like a Wal-mart but the clothes are better quality.
I've also found lots of decent stuff at Target -- look for their "yoga pants" and solid matching tees/long-sleeved tops etc. They do sell a lot of decorated stuff but you can get a lot of decent-looking things there too.
Penney, Sears, etc. - too much junk and yes, too much stuff that looks "tarty" on little girls. I hate Justice and Limited Too etc. for the same reason and so does my daughter. Pure junk -- every shirt just has to have a design or peace signs or words on it, and the quality is so lousy and flimsy. I feel for you!! But stick to your guns and your daughter will pick up good and modest tastes from what you get her now. My daughter is 10 and does not want anything to do with tacky tees and short-shorts like she sees on too many girls at school. She now chooses what she likes, and she wears a lot of soft, long trousers, khaki trousers, skirts over ankle-length leggings, polo-style collared shirts, and soft solid-colored nice tees (mostly from Target) that she likes to dress up with belts or layer with light jackets. A girl CAN dress neatly and nicely without either being in a uniform or going the route of low-cut on top and high-cut on the bottom!
One of my daughter's friends last year (fourth grade) came in and announced that she was giving up asking for clothes from Justice because she realized she did not like advertising for a store by wearing tees covered in its name, or wearing symbols etc. that some store thought were cool. My daughter was really impressed by that and so was I!