The best shirts are made by custom tailors or custom shirtmakers.
Buying a premade shirt and then having it ripped apart for a fitting is backwards thinking.
Custom/Bespoke Tailors are not the same as alteration tailors. Some alteration tailors are also bespoke tailors who prefer to do alterations and some do both. Some people do not even call alteration tailors tailors. Real tailors know how to make garments while non-tailors alter premade garments. Some tailors don't know how to make shirts. Some custom shirtmakers call themselves shirt tailors. And, of course, skirt making skills vary from maker to maker. And don't forget, manufactures are not tailors. Salesmen are told what to say about the products in the store- the person who tells the salesman what to say isn't a custom shirtmaker either, so you can't expect much from them.
A real shirt maker or tailor who makes shirts starts with asking the customer what the shirt is for. And then the pattern he makes is designed around the purpose of the shirt. There is also questions about what Cloth the customer would like for that shirt. Shirting, as with many types of cloth products, has several types of weaves to choose from. So this is something to ask about if you are unfamiliar about weaves. Many shirts are made of cotton and some of linen or silk, once in a while wool, or something else. Not all custom shirts cost more than premade. Once he has a personal pattern for you other shirts you want made are simple to make without all the refitting problems (this can save you time and money).