I shop a lot at vintage stores, get some nice deals. I'll also go to H&M, Nordstrom Rack, Loehmans, Macy's, Armani Exchange, French Connection, etc.
I find most of my favorite shirts are either Ben Sherman or Original Penguin. For jeans I prefer Citizens of Humanity or just the cheaper H&M jeans. I've got some Chip and Pepper, D&G, and others in jeans, but I don't know, they aren't much better than the H&M's in terms of quality. The Citizens are nice, I'd buy another pair of those, quality is pretty good and they're nice an long for their size.
For dress shirts you can't go wrong with Hugo Boss, Armani, BCGB, etc, lots of good brands out there in that area. For ties I like Christian Lacroix, good prices if you look hard, nice patterns, nice weight and feel. For pants, go pleat-less, no question, don't even think about pleats if you have a choice. Calvin Klein makes some pants that can be had for very little, they fit well and look good for about $40 or maybe less if I recall. Get a good pair of dress shoes as well, look around, you can get a fairly decent pair for under $75 if you know where to look. If you've got the money go Canali or Brioni all the way (but thats assuming you've got about the amount of money a used Civic costs to spend).
Do yourself a favor and invest in a nice belt. I think its goofy when someone's got nice pants on that are falling down because they couldn't afford a belt.
Jackets and such you can really find anywhere. I've got a sweet H&M wool double breasted coat that I love. Also have a nice Dolce and Gabbana sweater that I love as well. As well as a nice American Eagle sweater that was like $15 (not as nice as the D&G but far cheaper new).
Look for a good vintage store in your area, I've gotten so many shirts that must have been worn once and then sold to the store. I've seen Givenchy, D&G, Armani Collezioni, Yves St. Laurent, etc, all at American Eagle prices at my local vintage shop. Depending on where you are it might be hard to find one this good, but its worth a look.
If you're into designer stuff
BlueFly is a good place to start.