Going by the look of the market window you are using Cipsoft client so either you have broken Tibia.dat or Tibia.exe because as I already said categories are handled client-side and are hardcoded inside Tibia.dat.
You can search in server source but you wouldn't find so it's only waste of time, as I said already find different source to download client or you will end up searching forever for solution.
p.s. for 11+ clients it is the same but it is inside appearances asset file.