I like this last one best Norman -- great find! Sorry I missed this thread earlier.
For readily available materials, here in the U.S., that means a ball point pen, liquid shellac, and denatured alcohol (shellac thinner) will do the job. The last two are available in every hardware or paint store here. I have both on hand for woodwork, including pattern making, anyway.
Dry Prussian blue pigment, while available in specialty city art stores, is not something locatable in my town, and probably not cheaper or easier to obtain than Dykem.
Prussian blue in tubes of oil paints is locally available, but also expensive. I'm not sure how well it would mix, or how quickly it would dry in alcohol. Probably not as well on either count.
Prussian blue oil paint It will work in a pinch as a spotting marker for scraping, but not as well (it's paler and dries more quickly) as the "real" thing -- I've used both.
Norman's find....a used ballpoint pen tube? Found everywhere, free. That's the kind of tip I really love.