For a dressmaker, Pat's Custom Buttons and Belts in California in one of the greatest resources EVER. You send her a piece of your fabric and she will make fabric covered buttons or belts in a host of styles, shapes and sizes to perfectly match your garment. I sent off a scrap of fabric from my client's wedding gown and received these in the mail today:

Not only did I receive an exceptional quality product, she sent me five extra buttons for free, gave me a discount on top of that, returned the unused fabric and the total turn around time (I'm talking from the time I mailed the fabric to when I had buttons in my hand) was just four days. Four. I can't even answer emails in four days. Pat Mahoney, if you ever read this, you are my hero and I beg you, please, do not ever go out of business!