Stand back, y'all. I got this one. :)
Shirts
I used to hound the thrift stores for dress shirts, because I would actually get some good stuff for a couple of bucks every 1 trip out of 5 or so. These days I just hit up Ross because (through trial and error) I found that I can find a shirt there that fits me almost perfectly for $12. Poke around the brands there and you can probably find something that suits you.
Shoes
If you can get over the "used shoe" thing, eBay is a fantastic resource for dress shoes. I got my favorite pair of loafers there for <$30. Just make sure you know for a fact that a particular brand/size will fit you. Don't count on returning them.
Suits
eBay is the best resource here, too. Take your measurements for chest, sleeve, and shoulder (at a minimum), then set aside an hour or so every couple of weeks and start hunting. Sadly, eBay doesn't provide for listing or searching by detailed measurements, so you'll need to narrow by the closest 2 or 3 off-the-rack sizes and then read the individual descriptions for anything that looks good from the thumbnail. You can get good all-season wool suits this way for $100 or less. Sometimes much less. (Yes, you could maybe get a polyester Club Room suit or some other such crap for around the same, but...don't. You'll look like a rube, and that defeats the purpose of wearing a suit.)
If you feel yourself considering dropping $200 to $300 for a Jos. A Bank or something, consider that you could have a suit custom-made for you at that price on any number of online tailors such as Modern Tailor or Studio Suits. Honestly, there really is no reason to pay that much for an OTR suit these days.
Hope that helps!