I’m a fan of Brooks Brothers Extra-Slim Fit OCBDs (their dress shirts are fine, too). They’ll probably be on sale soon though, so maybe sign up for their mailing list. You can generally get these for around $50/shirt during their Friends & Family sales or during certain item-specific deals they’ll run.
Run of the Mill Shop has their shirts starting at $130. No personal experience with them, but if you fit within their sizing, that might be an option.
If you have a perfect-fitting shirt already and can be bothered to head to your local fabric and craft store to buy a tailor’s measuring tape, then I’d suggest going with online MTM shirting. BiasedCut has some great shirts starting at $85. There’s a lot of other services, too, so you can do your research on that.
A bit higher up the price range would be in-house shirting from Wharf and Epaulet.