If you have a RTW dress or sports shirt that you like but is large across the chest and waist, here's a Threadbanger (what a name!) DIY video on altering a shirt by taking in the sides:
Remember pinch and pin, pinch and pin and sew. Obviously, I wouldn't recommend this for your $600 Anna Matuozzo shirt. But as the video suggests, if you are serious about making shirts, there is a very good book called Shirtmaking by David Page Coffin, which my own shirtmaker Freddy Vandescasteele recommends.
The video also features Bodymetrics, a body measurement company. Bodymetrics is focused on using its bodyscan for fitting jeans (apparently 25-30% of all garments are returned due to wrong size).
A cheaper alternative to Bodymetrics is creating a bodyform using three tools - a duct tape, a t-shirt and a friend. Seems much more commonplace in constructing women's clothes. It would be interesting to know of any men's tailors or shirtmakers who take this approach.