Hi Bryan, any toy store hobby shop... it's just testors enamal model paint... The ivory suggestion has merit also, and as inexpensive as the paints are... you might want to try it also.. I used white and yellow... and just used drops for trial and error... (example: 20 drops white, 1 drop yellow, mix, mix, mix!!) etc... I think originally they just used white.. the off white you're trying to replicate is just a century of aging!....let us know how it works out... remember, time and patience...
...best to you, til...lat'r...GT
Oops! almost forgot! You really have to pre-clean the area to be filled.. acetone works well...