In the states, Goulet Pens is a wonderful niche retailer of inks and fountain pens. https://www.gouletpens.com/
Though many people avoid Goulet due to the anti-LGBT stance of their owners.
Strong recommend. Brian's a friendly and diligent guy, always ready to help research and solve customer problems. The only fountain pens I don't get from Goulet Pens are the ones they don't sell.
I've been buying from Goulet for years, but lately their service has taken a major nosedive. I recommend people stay away from them now.
The last pen I received from them was broken. They would not take it back, asked me to deal with the manufacturer directly. Horrible customer service. After so many expensive pens (Pelikan, Pilot and Visconti), I won't be buying from them again.