You can be a sly old fox!
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Or suit your fancy -
I'm glad someone linked to Benson and Clegg. Some fine choices there, even if only to provide inspiration.
On buttons, there is nothing wrong with brass - it is a sound and traditional choice (although I wouldn't have monogrammed buttons as a personal choice). Brown horn still signals "odd jacket / sport coat" rather than "orphaned suit jacket", and is a more understated choice. Silver or pewter can work in the right circumstances. Textured metal more pleasing in my view than flat and shiny.
On the orphaned suit jacket issue more generally, a plain navy suit jacket can generally be redeployed as an odd jacket. But cloth will play a part in this (i.e. a traditional blazer cloth, in hopsack, flannel or twill texture will look better than a smooth worsted in my opinion). Some will say an odd jacket should always have patch pockets. I don't care for patch pockets at all (perhaps on a light weight, pale summer jacket), and for my part think this sort of advice can be safely ignored. Certainly there are plenty of examples of traditional, well-attired chaps in blazers without patch pockets.