I have had similar difficulty at stores like Banana Republic and the Gap. Brooks Brothers is a good choice if you are going for a very conservative, traditional look. Their clothers always conjure up images of Wall Street investment bankers for me. Maybe great for business, but not necessarily appropriate for an evening out.
If you are looking for casual street clothes, I've had better luck at Structure and Abecrombie & Fitch, both of which are national chains. Some of their lines are designed for bigger, muscular men.
But ultimately, the issue isn't just what they wear, but also how you wear it. First, make sure your clothes are ironed or folded. It doesn't matter how great your clothes are, if they are wrinkled and dirty, you will look like a slob. Second, a great shirt and a great pair of pants don't necessarily make a great outfit. If you are unsure about how to mix and match, look at what the store owners put on the mannequins and buy the whole ensemble. Last, don't pull your belt down below your waist -- it just makes your gut look bigger. Why straight guys do this I'll never know...