Brooks Brothers has nice shirts, but you've already spent a substantial amount (how much was your suit and shoes indepently?) so I'm not sure what you're looking to spend. They're $75 each or 3 for $189 for the non-iron type (which I've read alot of "fashion obsessed" people dislike..but I think it's a nice thingto throw it in the dryer and 10 minutes later have it look like it's freshly pressed. The non-non-iron are the same price, and I think they might be a little bit less in outlets (you have an outlet in Sante Fe, and a retail store in Scottsdale, AZ if you're ever in either of those areas.) Also, I know if they don't have your size they'll order it for you and will overnight it at no charge.
However, if I were you (although I do like BB's shirts) I would order from Land's End. Their shirts are said to be really nice for the money (sometimes said to be better than the BB outlet line, "346") and they cost from $19-$25 dollars. I know you can get them in Sears, but I'm not sure where else. You could order it online and have it monogrammed there to add a personal touch, but I guess monograms aren't for everybody.
Oh, but remember hat a button down collar is less formal so get at least one regular color (I guess white) for nicer things.
But yeah, most places have dress shirts...including Wal-Mart.
Oh, I was able to get a lot of information on this type of stuff before I bought my suit from the
Ask Andy Forums. Maybe it will help you out too.