I'd completely forgotten about this (which is funny because this is my topic), but when I went through Air Force OCS back in the early 80s we had to mark all our clothing with ink.
We were to use black ink for light clothing and white ink for dark clothing.
The black was simply India ink, but the white stuff had to be purchased down at Uniform Sales.
In addition to clothing we also had to mark all our personal property in our room, and I used the white stuff on all sorts of
surfaces.
I think we all bought a kit like this one from Vanguard:
Marking Kit: Com-Pac (SKU: 7081300)
Obviously we don't need the whole kit if we're using the DoD stamp, so I did a quick search at Vanguard and found this:
Uniform Marking Kit: White Ink (SKU: 7080150)
I looked at OfficeDepot and all their stamp pads appear to be pre-inked. My local stamp-maker has pads but no white ink.
If you don't have a shop like that nearby, Vanguard has a
Uniform Marking Kit Pad Refill kit that includes a stamp pad and a bottle of black ink, but the pad appears to be dry so you could probably use it with the white ink.
I need a couple of uniform items from Vanguard; I think I'll add a couple of bottles to the order and see how it works.
If the white stuff from Vanguard doesn't work I'll probably just use flat white model paint.