Any Singer or even Greist button hollers ( Greist made the Singer ones ) for a low shank will work on your 99K.
The Singer ones come in the green rectangular and egg shaped boxes. The older ones come in paste board boxes and will most likely be for the short shank as they went to the plastic boxes before the slant shank machines came out.
And yes they do make the most bestest button holes.
Oh, if you want bigger button holes get a low shank Professional Button Holler kit. I really like those.
Joe