It might depend on the style and construction of what you have, but for the most part, any tweed waistcoat will probably be fine for wearing with a kilt. To make it more "Scottish", you could swap the buttons out for antler buttons, and add pointed flaps to the pockets. But that's not really necessary. The main thing I would do first is see whether the bottom front panel needs to be adjusted in height so it works with a sporran. I've found that most Saxon-style waistcoats come down too far in the front for sporran-wearing.
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