ACE Hardware sells the number/letter sizes in individual packs at a decent price. They're on the same rack as the taps/dies and the taps say right on the package what drill bit to use so I find it easiest just to pick up the individual drill bit when I need it, and then I've been storing the drill bit and the appropriate tap in the same envelope for next time. Much cheaper than buying an entire set and using 2-3 of them in a lifetime.
Joe
Joe