Egg safe ink is basically a food grade ink that's safe to use on eggs. You can buy it at a lot of places. Including Amazon and Etsy. I have red and black. But they have other colors too. Just make sure it says either food grade ink or egg safe ink.
I use an egg safe ink to date them the day they were laid.
I bought 3 customs stamps from Etsy.
1 stamp has my "farm name" and all the things the state of Taxachusetts requires you to have on your egg cartons if you choose to sell them.
The 2nd stamp has egg storage and handling...