Some basic info you may already know. 99s use class 66 bobbins and standard 15x1 needle available pretty much everywhere. One thing I've noticed is new class 66 bobbins (the ones marked Singer at places like Walmart and Joann's) are a hair narrower than vintage ones so may not fit your bobbin winder as well but will work ok inside your machine once they're wound. I haven't tried bobbins from other places so I'm not sure if they're better or not yet.
Rodney