The tutorial I will be writing will use a free kit as its subject, but supposing you wished to buy a paper model kit, where could you get one, and who makes the best kits?
The first question is rather easier to answer than the second, so we'll start by discussing the various publishing houses. Few card model designers run their own publishing outfit; usually the designers farm their kits out to one or more publishing houses. This makes it a little difficult to generalize about Publisher A versus Publisher B, because a designer might have his or her design published at both places. Lesson: It pays to take note of who designed a particular kit you like, then look for that designer's work at different sites. Something else to make note of is the date of publication. CAD designing did not become commonplace until about the turn of the new century -- the earlier a kit is published prior to 2000, the more likely it is to be hand-drawn. This is not to say that all hand-drawn kits are bad (they aren't) or that CAD-drawn kits are always preferable (they aren't either), but CAD-drawn kits usually have more and finer detail.
With that in mind, let's take a look at some of the major publishing firms. This is by no means an exhaustive list, because due to the nature of the product and the fact that it can easily be electronically distributed, there are dozens of Mom and Pop sites that may have a limited offering available. Here's a few of the better-known outfits:
GPM: GPM are one of the largest card model publishers in Poland. They have a large selection of ship models in many scales, both in their own line and from many other publishers as well. They also have an extensive inventory of after-market accessories. Ordering by mail from Poland is usually not terribly expensive. GPM's listed postage rates are steep, but according to reports from other modelers, GPM will calculate the actual postage charge and refund the difference between that and the rate calculated at their website.
Modelik: Modelik carries only their own line of models, but as discussed previously, these are not all the work of one designer. Modelik kits are generally well-regarded in the hobby. Modelik charges a flat shipping rate of 15 EUR per order.
JSC: JSC have an extensive line of ship models and are one of the few Polish publishers to offer a large selection of models in a scale 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. JSC sells directly to the public and through other on-line retailers.
Kartonowy Arsenal (KA): Better known by the surname of its owner and chief designer, Halinski, KA offer highly detailed models of mostly WWII warships. Though often considered suited only for advanced modelers, the fact that they usually fit together so well eliminates some of the problems found in less well designed kits. KA does not sell directly to the public.
Orel: This Ukrainian publisher is relatively new to the scene, but they have been prolific. They offer a large selection of mainly Czarist-era Russian vessels, including torpedo boats, destroyers, cruisers, and pre-Dreadnoughts in 1/200 scale. Orel sells directly to the public as well as through larger firms like GPM.
HMV: The Hamburger Modellbaubogen Verlag is probably the premier German ship model publishing house. Like other German publishers, their kits come in the 1/250 scale preferred in Germany. They have a wide selection of mostly German warships, passenger ships, and merchantmen. They also offer an extensive line of after-market detail sets. HMV have been good about upgrading their designs, and several of their older hand-drawn models are currently being reworked as CAD models. HMV does not sell directly to the public from their website.
Moewe Verlag: 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 hand-drawn models dating as far back as the 1960’s, and this isn’t always obvious when browsing their website. MV kits tend to be less detailed than HMV kits, but they have a broader selection and somewhat lower prices. They don’t offer after-market detail sets. Moewe kits can be purchased on-line from their website.
J F Schreiber: Schreiber are 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. Schreiber operates an on-line store.
Paper Shipwright: Designer David Hathaway’s line of kits is unique in its extensive offering of monitors. David also designs small passenger craft and working vessels. These are generally very good kits and also reasonably priced, with a fair but not overwhelming amount of detail. Paper Shipwright kits can be ordered directly from their website.
In addition to the aforementioned publishers, many excellent designs are available from smaller outfits, some notable ones of which include Golden Bear Models, Digital Navy, WAK, and Answer (the latter two are not exactly 'small outfits', but their ship model selection is limited). As noted, some of these publishers sell directly to the public, and some don’t. Finding a retailer that carries the kits you want can sometimes be a taxing ordeal, but here are some of the more popular sites:
Karton Modell Shop is a German retailer with a good selection from a variety of designers.
Marcle Models are one of the larger distributors in the UK.
Paper Model Store is practically the only US retailer with a broad selection of Polish kits. None from Germany, though. Paper Models International used to be a well-stocked and reliable supplier, but they were sold four years ago, and their website hasn’t been updated since then.
GreMir Models offer kits in either CD or download form. Many of their models, which must be printed by the buyer, are the same models offered in print by WAK or Modelik.
E-Cardmodels is a retail venue for a slew of small designers -- a lot of variety of subjects, scales, and degree of difficulty.
HMV operates an on-line store at Amazon.de.
There are many, many other places to buy kits on-line, but these should get you started. Have fun browsing!