First off, I’d avoid doing the whole S/M/L sizing thing and start buying shirts meant to fit your exact neck and sleeve size. Brooks Brothers does have OCBDs that come in exact sizes.
If you don’t know your sleeve and neck sizes (or any of your sizes and measurements), then spend $2 and buy a tailor’s tape and have someone help you take measurements of your neck, sleeve, chest, etc. This will help you find out what shirts will fit you when you consult their sizing charts.
Secondly, you could try places like Howard Yount, Epaulet, Kent Wang, Sid Mashburn, WHARF and Gitman Vintage.
If you have a shirt that fits you decently, then you can reproduce it by taking measurements off that shirt and using online MTM services. Or you can use a place like MyTailor and meet with a tailor in-person to take your sizes. I’d give MTM a try since it’s about the same as retail in many cases and you get exactly what you want. Downside it turnaround time can be up to 8 weeks.
