Since I'm going to convention for the first time this year, and my white dress from college (which I loved) has yellowed a little with age (it's been almost 9 years) and is a little snug, I just ordered a new white dress.
PNMs, if you want to go the online route (not always great since if it doesn't work you have to send it back and you have less time), try eShakti. Often you can customize your dress to what your group requires. Naturally, you'll probably only want to customize for initiation if white dress is required, you won't know what you need for presents unless your university spells it out for you.
I ordered this dress from there:
(Direct link in case you want it:
http://www.eshakti.com/Product/CL002...k-poplin-dress)
I customized it so that instead of above-the-knee length, it'll be full length. I'll have to wear a white cardigan but I was expecting that with anything since sleeves are apparently sooooooooo 1950s.
They have a bunch of white options right now (summer), and they all look pretty cute. I've been happy with the fit and quality of everything I've ordered from there. You can also customize sizes, so, if you're usually a 4, but you're a 6 in the bust, you can take your measurements. I just fit into a regular size. Customization is usually $7.50 or so. Today it just so happens that they have free customization.
For us older ladies going to convention who haven't had to search for white dresses in 800 years, this was much less stressful!