This dress from Land's End should work for most groups:
http://www.landsend.com/pp/womens-su...&origin=search
It's knee length, and the shoulders and the back are covered.
This one from Macy's is very similar:
http://www1.macys.com/shop/product/s...4-_-34-_-MP434
I like this one from Eddie Bauer:
http://www.eddiebauer.com/catalog/pr...viewAll=n&pg=1
This dress from Old Navy might work:
http://oldnavy.gap.com/browse/produc...scid=898302002
You might need a shrug or a cardigan if the group's rules say that a NM's dress for pledging has to have sleeves.
This dress for Kohl's is on sale for $27:
http://www.kohls.com/kohlsStore/zbra...chet+Dress.jsp
I love the Lilly dresses but they're sort of short. I'm really tall and have long legs, though. A lot of dresses that are a little short on some people leaves half my thigh exposed.