I'm bored at work so I threw together a quick chart comparing slimness and quality of
dress shirts I've worn.
My go to right now is Simons Le 31 but I wish the quality was better. It's also hard to find anything that isn't a pretty loud pattern which is a frustrating.
Otherwise I usually head to BR (non-outlet) as the quality is good for the price, but there are slimmer retailers out there.
Frank and Oak on 17th is worth checking out as well. Fits slightly slimmer than JCrew and BR but their sleeves can be a littler short.