I was just wondering, is there a general rule of thumb as to where you'd want to start looking for a specific ship? Say I want a certain speeder, how do I go about finding it? I know that certain companies design certain products, but don't the products get passed around npc shops?
NPC shops are not used for that at all; they only stock a few basic items like staves and knives, that are not produceable. The factions producing the items you want are a good place to go (check their websites), or you can check the player-created markets. All of that information is found in the Trading section of the Guide, which is linked at the top of each page.