If I need new affiliate program, then I go out looking for sites. If I am satisfied with current then I don't.
Generally affiliates are lazy people. If they are happy with current affiliate program. They don't want to keep changing links. So keep them happy and they will be yours for lifetime :)
And yes, if they are happy with you then they will refer other affiliates, even if you are not paying any commission for referring them. And most of these new affiliates also generate sales. So you will get more active affiliates without doing anything extra.