Housing Works have a good selection online - most of which you can buy immediately. Each of their New York shops has a fantastic window display - and all the items are on auction -which you can check out and bid online.
To illustrate my point of good thrifting online, check this out:
A beautiful vintage silk chiffon green dress, perfect for spring. I love the empire waist and the ruched bust. This beauty is in an auction - ending 26th May. It starts at $35.
I would pair the dress with this investment piece (read expensive but would wear over and over again) - a Ann Demeulemeester blazer. I love the combination of the feminine floaty dress with a killer tailored blazer. Ann Demeulemeester is a Belgian desginer well known for her perfect cut, edgy monochromatic clothes, which are as beautiful as they are horribly expensive. The thrifted blazer is $175, is in perfect condition and is available online right now. Unfortunately it is too large for me but I would love this in my wardrobe - it would look great with anything.