Website for getting bike-specific touchup paints?
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. :)

http://gunnarbikes.com/site/order/touchup/

ColorRite also has a wide range of paint touch up pens available. 

http://www.colorrite.com/product/touchup-pen-primer-1071.cfm