My first favorite is Amazon.com. It is actually the very first on-line store I felt comfortable ordering from. I don't think they need any promotion because everyone knows them already.
Recently, I discovered another store called MikaRose.
MikaRose is an online a store with a wide variety of modest but very elegant dresses. When I first discovered it, it reminded me of the "Moms of Modesty" campaign on blogosphere (which reminded me that I need to put the button back in my sidebar). I thought, since I am promoting "Moms For Modesty" I should be a role model for modesty for my daughter - this has became one of my favorite on-line store. And because the owner was quite pleased with all your visits last Bloggy Giveaway time, she said that she's giving 10% discount to whoever buys from their store from this post. They also have a promo of Free Shipping if you order 2 dresses. There's still time for Valentine's Day so go Mikarose.com now to buy a modest but elegant dress for your date :)
And for more recommended on-line stores, go to Rocks In My Dryer.