We used Nanny Mall (
www.nannymall.com) to find our nanny. This is an organization local to the St. Louis area. We found their fees to be significantly lower than other placement agencies. (Probably because they are a small organization working primarily out of their own homes and therefore have less overhead than others.)
They have about 50 nannies with them who are looking for a variety of jobs (part time, full time, temporary, permanent, etc.) We wrote up a description of our open position and the staff gave us the portfolios of 5 or 6 nannies who were interested in the position and whom they felt was a good match. I did phone interviews with 3 and an in person interview with 1, whom we hired right away. The whole process was complete in about 2-3 weeks. We have been very happy with her.
I should also say that Patti at Nanny Mall was very helpful to us about describing and forming the position itself. (We had never hired a nanny before and were not sure what was customary as far as payment, vacation, sick days, etc.)