Sawyer Brook has great buttons, and they're really nice folks - if you email them with a picture of your coat I bet they can give you suggestions. I could give you other recs for fabric stores that have good button options as well - Britex would be a good option for higher-end stuff too.
Otherwise if you have an independent fabric store in your area it might be worth a look. (IME Jo-ann's, Hobby Lobby, etc., don't have a wide selection, especially in larger coat buttons.) Here in the SF area Stonemountain and Daughter has a great button collection at their store, but I don't think they have them online. Sometimes the difference between cheap and expensive-looking is all about the sheen/material, which is hard to determine online.
As re style choices, matching red buttons would be ideal but might be tricky to find in the exact right shade. You could do black if you wanted, or a metallic that's a little less shiny than the original ones that are on it. Or a tortoise-shell look would be very chic.