A while ago I came across a website that did touchup pens for a catalogue of old bikes, but I can't seem to find it now. I have a specific old bike that I want to check if they have, before I go to the auto-shop.
Gunnar bikes have a selection of touch up paint for classic bikes.
They also have great information on how to use it, and what results can be realistically expected. :)
ColorRite also has a wide range of paint touch up pens available.