I'm not an expert on formal Scottish wear, so take my suggestion with a grain of salt. I have had very good luck with Cold Steel products. I don't own one, but I have been eyeing their Scottish Dirk, which is styled like a traditional dirk. They can be had for around $90 at a lot of different places. The quality of what I own has always been very high.

https://www.atlantacutlery.com/scottish-dirk