Where you should shop depends on your physique. My shoulders are a good bit wider than my waist so it makes it difficult to buy off the rack and have something that fits correctly. Even the most expensive suit will look like crap if it doesn't fit.
I had a suit made for my wedding by these guys in Atlanta
Guffey's - Atlanta's Finest Specialty Store for Men, it wasn't as cheap as one of the warehouse places but I can put my arms out without the jacket cuff coming up to my elbow.