I find eating a balanced diet in general is good for focus, not necessarily a specific food. Lots of leafy greens, healthy fats, keeping carbs restricted to some extent without taking in any added sugars and junk. Vitamin D supplementation, etc. I have heard positive things about Lions Mane and Ashwagandha helping with focus. I personally tried a Lions Mane supplement and felt nothing.
Mineralization Mineralization Mineralization.
A large portion of my diet is raw/poached eggs, red meat including organs, raw milk/cream/butter (milks and cream is relatively easy to buy, butter you make yourself) with occasional fish. This covers virtually everything I need.
Unrefined sea salt in your water.