Platos Closet is the cheapest option, but they definitely make sure to only take the "hot" designers/styles. So check & see if there is one near you.
If you're not in a hurry (don't need tons of clothes fast), look at all the local thrift stores with her. I've gotten Jones of NY & Ann Taylor stuff at the thrift stores near us, but it was just because I had time and could look through the items (I'm not fashion maven--I just happened on the items & bought them, and looked at the manufacturers tags after I got them home). And it pays to check them often for new stuff, so if any of them are along your regular errand or shopping route, it's worth it to stop & take 10 or 15 min to run through the store.
Also, check your local yellow pages for "second hand" or "thrift" stores--I think you might have better luck at the mom & pop stores than the VOA or Salvation Army or Good Will.
You might also check http://www.kidsconsignmentsales.com/ for sales near you. I've been to a couple of consignment sales in Ohio, and there are always tons of items that range from the barely useable (that is a small percentage of items) to the designer/brand name top of the line stuff. Usually the top of the line stuff is the first to go, though, so for these, I would shop early at the beginning of the sale.