Some artists are perfectly willing to do sketches for no consideration. Be very nice to these people. They think that having other people see their work in your book will be reward enough. However, just because they do it for someone else doesn't mean that they owe you anything. It's perfectly natural for an artist to give preferential treatment to his friends.
Themes can be fun, if they're flexible.
You especially shouldn't nag, be anal, or hover over the artist making criticisms before he's finished. Remember that fun is the main consideration, and if the artist enjoys doing the drawing, he is more likely to do a good one, and less likely to ask for anything. And if an artist turns you down, accept it. He may be tired of drawing, may not want to do your theme, or he may have some other reason. If he has a huge stack of books pending, he's probably doing you a favor by letting you go to someone else first.