Noodler's inks are dime a gallon, esp. in the US, and they're guaranteed to be pH neutral in most used colors (bottles state if they pH neutral), so they don't damage any pen, modern and vintage.

They're specifically designed to be that.

Bulletproof Noodler's inks are as follows: https://noodlersink.com/product-tag/bullet-proof/

They even have 16 Oz. bottles if you want "buy one, leave for grandchildren" amount of ink in one go.

If you want a more mainstream archival ink, go GvFC, Pelikan 4001 Blue Black (not available in US), MB midnight blue or Pelikan Edelstein Tanzanite.