I bought my first fountain pen on Amazon (didn't know about Jetpens/Goulet/others).

My second pen (Jinhao) - I think I got it from eBay.

I suspect my next pen was also from Amazon.

After that, though, all my pens came from niche stores. A number of PenBBS pens directly from the maker, etc.

I just compared one of the pens I bought: Amazon is not cheaper than Jetpens.