I work with a lot of fighters, and there is no one supplement or group of supplements that are designed specifically for fighters...
However, there are some supps that most fighters will use on a daily basis.
Bcaa's are highly effective, both for periods of intense training and periods of having to make weight. A multi vit, green powder, and protein supplement will benefit even the best of eaters...especially when considering the schedule that many fighters have to keep.
Depending on the individual, one may take things for certain injuries, inflammation, exposure to virus or the like from training partners etc.