My wife and I are ready to start trying...but how?

We have two kids. The oldest was from a previous relationship. My wife came into the picture when he was a year old (he's five now). Our second one we had with the help of my ex (the father of my first). But this time, my wife wants to be the one to carry. She does not want his kid. Things with him were rocky for a while, so she wants someone who won't be in the picture. How the heck do we do this? We don't really have anything saved up. We live fine and can afford three kids, but not a whole lot more. It's not like we have hundreds to throw away on getting inseminated and whatnot. Any advice from those of you going through this?