Let me dust some of the bile off of the crusty black thing where my heart used to be and impart a little artistic wisdom to you. Especially since you sound a tad like a fellow with Aspergers so I can kind of relate to your social awkwardness.
As a person who's a huge whore for art and has acquired literally hundreds of pieces over the years... Here's how it works.
Don't ask for free artwork at random. Ever. It is the quickest and surest way in the entirety of all of the history of man to look like a mooch. Just doesn't work that way. If you have an artist that's your personal friend, it's s slightly different situation.
Generally it only works like this. You pay an artist, and give them a detailed description. They draw. They usually show you some rough sketches and you ask for a few moderate changes if necessary. The artists shows you again, and when approved off you go. (Don't abuse this.)
Rates can be extremely reasonable depending on what it is you want. Try getting to know them as people as well, usually artists are some damned interesting people. Did you know Vektor runs a gay night club? He's not gay himself, but he just prefers the drinks and music. (I kid, Vektor, please don't have the Enterprise F bombard my house!)
I don't know many people available that do Star Trek type vessels, except my own buddy Jeff.
http://stourangeau.deviantart.com/ This fellow is active on the forum as Titancreator, I believe. He does some pretty good stuff for about eighty bucks, which for the quality of his work is a damned steal. He'd have more details.
For my RPG, New Horizon, I've had to do this hundreds of times, with some commissions costing thousands of dollars, so if you're willing to own up to the notion you need to pay for art, I can offer a few tips on how to approach artists and prepare a solicitation.
