If you're asking how he got those custom made ones with a trident type blade, then the answer is probably the same people who make normal kunais. Since he was the Hokage after all, he probably had enough influence to have those custom made.
If you're asking how he gets them all on a battlefield, then it's the same way Ten-Ten does it. He summons the amount he needs each time, then throws them. He has to prep all these kunais before battle with the FTG marks on them. They don't just create themselves. If they run out, he has no choice but to pick up some he already used or stop using FTG.