Er, order them for the 999.
Reference is 50612- (_) = insert here whatever you want in number of teeth, from 36 to 39.
This is for the standard steel sprocket, in 525 chain pitch.
If you are going racing, or going to 520 pitch chain, the sprockets are available in anodised alloy, ref; 51613-(_), from 36 to 43.
900SS sprocket reference is 50602, with 6 holes, in 520 pitch, whilst the 999 has 5.