I also mostly buy from Jetpens when I buy online. My partner likes Atlas Stationers, and I've also had good experiences with Goldspot which actually has sales every so often.

What I would also recommend is to look locally. Even if there's not an actual fountain pen or stationery store, in my experience a good amount of independent art supply stores will also carry a decent amount of fountain pen supplies.

Goulet doesn't seem to offer much unique except their social media presence.