There are many many sites about vintage fountain pens but here is a little selection :
Morastylos: André Mora is a great french specialist of fountain pens. You will find on his site modern and vintage pens for sale as well as his brand "Oldwin". Si vous avez un stylo à réparer, n'hésitez pas à passer le voir, 7 rue de Tournon à Paris.
Vintagepens: David Nishimura web site. Another great specialist of vintage pens. Un puit de science ! You will find vintage pens for sale on his site but also many information on vintage pens, brands, systems, history ...
Lbepen.com: Mark Hoover has been working in vintage pens for years, mainly selling on penshows. Many vintage pens for sale on his site, american but also french pens which is quite rare on american sites. Mark has also developped his own products to restore vintage pens. The famous Deoxydizer to restore hard rubber as well as polish pastes or a balm.
atelier-lesoon.com: Thierry is french. He repairs and sells watches and fountain pens. Have a look at his site from time to time. He always have interesting things for sale.
Richardspens.com: Richard Binder is a great american specialist. You will find a lot of information on fountain pens on his site. Look the section "Reference Pages".
GregMinuskin: Greg is a specialist of Nib retipping. He is a very nice person, he works perfectly and very fast.
A Parker Collection: This is the blog of a Parker collector. He shows his pens (in french) with pictures, humor and stories of his pens. Nice collection, all sold now.
Pentrace: Two boards on this forum. The green board is the sale / trade section. The yellow board is the forum for discussions on fountain pens. On sunday, during the "Pentrace Sunday Forum" (PSF) pentracers post pictures of fountain pens on two different topics. A little family.
Fountain Pen Network: A big forum on fountain pens with several different sections and many members.
Styloplume: A forum in french. Good to find information for french collectors. Very friendly.
There are also several groups on Facebook where collectors share information and findings every day.