Drop into any Land's End store. Lots of modest clothing in there. Really.
Or Eddie Bauer. Same sort of thing.
You're not going to find it at The Gap or Wet Seal or any of those stores, I guarantee.
When you start seeing modest clothing in Seventeen and the other fashion magazines aimed at young girls, then you'll have no trouble getting girls into that clothing.
Around here, in Minnesota, there's a chain called Mills Fleet Farm. It's basically a big, old-fashioned feed store writ large.
They have clothing for all ages, and it's all modest. Could be there's something like that, too, in your area. Convincing your daughters to buy their clothing there, however, is your responsibility. I wouldn't even try.
Pricey, but worth it.
Personally, I have the opposite problem. Lands end and Eddie Bauer, as well as a lot of the classic places carry only misses and women's sizes. Being short I need junior sizes, and that makes it even more difficult.
The Gap?
I don't think many of the Gap's clothing as being particularly immodest.