This question already has an answer here:
this is an example of one of the filters - Camdiox 165x208x2.5
this is the filter holder - SFH-14 filter holder designed specifically for the Samyang 14mm lens.
I was put off this specific filter holder for my Samyang 14mm after discovering there are only 3 available filters for it from Cokin; but I've heard these Camdiox filters are also compatible? They seem to have a bit more of a variety too, it seems.