From pictures I've seen it looks like the Kiwaya ukes have friction tuners. If that's what you have then Allparts has these tuners to replace them. https://www.allparts.com/TK-7871-001-Gotoh-4-Nickel-ukulele-Keys_p_3996.html
If you want to upgrade to the Gotoh 4:1 UPT tuners then follow Jim's advice. The holes will need to be enlarged. If you already have UPTs then you can replace just the knobs with ebony ones. https://www.allparts.com/buttons
It may be difficult (nearly imposible) to find just the buttons. If your ukulele is new, you might ask the seller if they'd exchange them (buttons or tuners). Maybe give HMS a call. Otherwise, you may have to buy tuners. And yes, just the buttons may be impossible to find anywhere but Japan.
Good luck.