You can get the Nikko G nib here, as well as other nibs, pen holders and ink. They also have different papers. Very friendly and fast.
Has a sheer abundance of nibs. You can spend hours browsing for rare or antique nibs, or just assemble your practice kit. They also have pen holders and ink. Very friendly and fast.
Big online store for art supplies, also brick & mortar stores in big German cities. You can get everything you need for calligraphy at this place, with one exception: nibs. They don’t have a lot of them. But you can get papers, ink, markers, pencils, watercolors, etc. Shipping can take a while, but I’ve only had good experiences in the stores.
British seller for fountain pens and inks. You can get the flexible Noodler’s pens here, as well as a really big variety of (fountain pen) inks. Very friendly and prompt service, shipping to Germany went fast.
One of the big sellers based in the US. Has practically everything relating to calligraphy, even hard-to-find literature about penmanship and the possibility to get the Letter Arts Review magazine. If shipping overseas they wait for you to confirm the shipping costs, which I find very friendly.