Joker is using different knives. You can use knives from the film like Microtech Combat Troodon or Smith & Wesson Extractor 1600. You may not be allowed to conventions with those knives. You can replace the blade or make the knife yourself (and many cosplayers would do it for realism). For this cosplay, I feel OK with making my own knife. Joker from Suicide Squad has scene where he is laying in circle of knives. We can use it to say
He has obsession with knives so he has more then we could see on screen..