I wear dress shirts every day and am very hard to size, so this is an issue that is very important to me. I have discovered that you don't need custom shirts. What you do need is a short that is fitted to your body shape and measured to the exact inch rather than half inches (34/35) or 35/36) as are sold in all department stores. I literally cannot fit into a single shirt sold at Macy's and can barely find a size that fits at Sak's or Nordstrom's, even if I want to pay their outrageous prices.
The solution is Brooks Brothers' regular wrinkle-free dress shirts.
Men's Dress Shirts | Non-Iron Shirts | Brooks Brothers
They come pre-sized to fit extra slim, slim, regular and large body styles and fit to the inch on both neck and arm length. They fit me almost as well as my real custom shirts. Usually they're around $85 a piece, but if you watch the sales you can either get 4 for $200 or a significant percent off for buying multiples. They're actually a good value at regular price but they're a great value if you can get them for $50 a piece. My closet is full of them. I stock up when they're on sale and I hardly buy anything else, even for my sons as they enter the professional world. By the way, I'm a perfect 16 neck and 36 inch sleeve. It's almost impossible to find that combination anywhere else.
Go get measured by someone who knows what they're doing and try a few on to see what fits best. I'd be surprised if you needed to go anywhere else.