For Americans wanting to work as au pairs, there are several agencies you can find au pair jobs with. You can also find different Au Pair agencies on the IAPA’s home page (the International Au Pair Association).
Another comprehensive list of au pair agencies can be found over at Au Pair Clearinghouse. Each agency has also been reviewed on that list, so there is more information than just a name and link. Edina Stone, director of Au Pair Clearinghouse, has some of the most credible information for host families and au pairs, to be sure to check out her list.
In addition to the big agencies, there are hundreds of other smaller au pair agencies worldwide. The best place to start is to decide which country you would like to work in and then search for agencies in that target country. Just remember, you want to be sure that you are working with a bonafide au pair agency with several years of experience successfully matching au pairs with good host families. Read more about finding a legitimate agency here.