WAK: https://www.wak.pl

This is currently a favorite Polish publisher and shop, as they have the best shipping rates to the US

GPM:  https://sklep.gpm.pl/en

GPM is one of the largest card model publishers in Poland.  They have a large selection of ship models in many scales, in their own line and from many other publishers as well.  They also have an extensive inventory of after-market accessories

Modelik:  https://modelik.pl

Modelik carries only their own line of models, but these are not all the work of one designer.  Modelik kits are generally well-regarded in the hobby 

JSC:  https://jsc.pl/en/3-modele

JSC have an extensive line of ship models. They are one of the few Polish publishers to offer a large selection of models in scales other than the usual 1/200 favored in that country.  JSC kits are normally either 1/250 or 1/400.  Many of JSC’s kits are older, hand-drawn designs

Shipyard: https://model-shipyard.com/en/135-paper-models 

A publisher of sailing subjects in 1/72 and 1/96 scales. Shipyard kits are available in the US from Ages of Sail.

Kartonowy Arsenal (KA):  https://www.halinski.com.pl

Better known as Halinski, KA offers highly detailed models.  KA kits are best suited for advanced modelers

Seahorse: https://seahorse.pl

Sailing subjects in a variety of scales.

HMV:  https://www.h-m-v.de

The Hamburger Modellbaubogen Verlag is likely the premier German ship model source.  Their kits are published in 1/250 scale, the scale preferred in Germany.  They have a wide selection of warships, passenger ships, and merchantmen.  They offer many after-market detail sets

Moewe Verlag:  https://www.papermod.com

Also known as Wilhelmshavener, this publisher has an extensive line of warships, passenger ships, and merchantmen in 1/250.  Ordering from them can be kind of tricky, since many of their kits are older hand-drawn models, and this isn’t indicated in the product description  

J F Schreiber:  https://www.schreiber-bogen.com

Schreiber is the third major German publisher.  Their kits are, as a rule, less detailed than either HMV or Moewe, but they offer more kits of simpler subjects suited to beginning modelers.  They also offer more kits in 1/100 scale than other publishers and operate an on-line store.

Paper Shipwright:  https://www.papershipwright.co.uk

Designer David Hathaway’s line of kits is unique in its extensive offering of buildiings, as well as ships.  These are generally very good kits and also reasonably priced, with a fair but not overwhelming amount of detail.  

Orel:  http://papermodeling.net

This Ukrainian publisher is prolific and offers a large selection of Czarist-era warships, sailing vessels, and Civil War ironclads in 1/200 scale

World of Paper Ships: https://www.worldofpaperships.nl

Dutch ships, ferries

In addition to the publishers just listed, there are some e-commerce sites that sell card models in general:

Shipmodell.com (Hungary)

Karton Modell Shop (Germany)

Marcle Models (UK)

Super-hobby.fr (France)

Cardfaq.org (free, but older website that is not maintained)