Yoox.com, bluefly.com, and eBay are good online sites, but you really need to do your research first and know your size and some basic stuff about the brands you're looking at.
As for pants: flat front is "in" right now; though pleats are fine, albeit more associated with older, more conservative dressers. Suits tend to be conservative in styling since the vast majority of people buy them for business wear (e.g. any chain store like Brooks Brothers, Men's Wearhouse, Jos A. Bank), so if you really want flat front pants you'll have to look at more fashion forward brands. That doesn't mean you can only find flat front pants in over-the-top Versace designs, but it means you'll have to look at "designer" (and I don't mean the cheap CK, DKNY, and Perry Ellis stuff you'll get at Macy's) brands and be willing to pay a little more. Also, don't bother with Armani: 1) even their top line is incredibly overrated and gets by on name recognition, 2) all their affordable lines are just cheap licensed out crap that only bear the Armani name but have no real connection to the man himself.