Most entities you would want are produced by factions. These are then sold on. Best place to look is the factions that hold the DC for the ship/vehicle/item/etc you want, see what their website says, and possibly contact the leader or their forum for ordering.
Other places to look is the Commerce Center forum around here, or off-site places such as Centerpoint Market (see the Centerpoint Space Station faction for that). However, these things will generally be nowhere near your location. As for paying, youwould generally send them the credits and they will send you the entity. Just be wary for those few players that scam to steal credits.
You could always use IRC, located in the top right corner on the darkness; or download a client. IRC allows you to talk to other individuals which play this game, and make business deals.
JUST DON'T GET SCAMMED - Use a middle for untrusted traders/higher value trades.
Are credits a problem?
You could always join a faction, which would include a monthly wage. Where you would be working for someone/faction. You could also faction sit, which is basically joining a faction and getting paid roughly 250k but not having to do any work. Working is better in my opinion because it comes with a far larger wage :)
The combine also have a CP (Combine Points) system, located on your right panel where you see your character under your level. All you have to do is donate some Real Life cash or you can click the sites every day of the week and vote for SWC. This does not earn a lot of CP's but eventually you can buy things with them such as the following;
Once the White Scenario is over you get exp and CP, the amount depends on how good and frequent of a GM in you were.
However everyone here told you how to buy stuff from other players but if your say on your home planet still and theirs a NPC with a shop just talk to them, they will ask you to buy some of their stuff and you can accept or deny.
If you accept they show you their inventory and you can select what you want to buy and when it's from an NPC the stuff is usually like 1000 credits tops and very basic but good for someone starting out.
Just be sure to scan the room once you buy the stuff and pick it up, a common mistake new members make is buying stuff but never picking it up so they end up livng without what they bought.