M,M:
Two recommendations:
Revolve -
http://www.revolveclothing.com/index.jsp
They have lots of different designers. Probably my favorite online store.
Nordstrom -
http://shop.nordstrom.com/
They are a good place to get started and also a good place if you easily get tired of shopping, because they don't try to carry tons of stuff, they take a sort of point of view on various styles and choose a few of them to carry. The tradeoff, of course, is that your selection is relatively limited. Still, for many guys, it's way more than enough stuff to choose from. I took a quick scan and it feels like there's a better selection in store than online, but it has been a little while since I went last, so not completely sure.