An athame is simply a ritual knife. If you are following a specific tradition, that tradition may have specific requirements.
Otherwise, I have seen a large variety of tools used for the job. They range from a folding razor knife, to LEGO swords in a mints tin for a portable altar; kitchen knives, to tactical knives, to elaborate fantasy blades.
There is also some disagreement in opinion to whether the athame should be used to prepare ingredients, whether a separate knife should be used. Again, I would say test your own tradition, your own feelings on the matter.