There are lace speciality stores - e.g., see
https://answers.yahoo.com/question/index?qid=20071201193321AAM3xgzI have also tried backpacking laces - you can buy 144" sections. If the laces are nylon, you melt or tape the ends to get something to pull on, and thread through the wholes. Leather laces can just be cut, I guess.
But I currently use nylon round utility cord - 3 mm diameter, if I remember right. You can buy it at camping and climbing equipment stores, like REI, in many highly visible colors and color combinations.
https://www.rei.com/search.html?q=utility+cord&origin=web&ir=q%3Autility+cord&page=1I currently use bright orange, which looks like Halloween against my black boots. I like round, because it doesn't twist, and is much easier on the fingers than flat lace. But you have to get better at pulling it tight, and you have to double knot it, because round cord loosens more easily.
But each to their own. I think "traditional" figure skating pepple favor laces that are the same color as the boots. Some ice skating judges are traditional.