Try a hobby shop. They sell sets and individual bits. As for the drill number size corrosponding to jet size. I don't know. I thought the same thig and I think they are close, but I wouldn't go by that until positively confirmed.
I had to clean a bunch of crusties out of my jet to get running and I just slid bits into the hole until I had a nice fit, then I went up one size.
Then I went and bought a new jet. The original worked fine, but I wanted to get my ped to stock before any mods.