I haven't used this service myself, but know someone who did and had success (although she did "interview" a few people before she found a sitter that she's went with)
www.sittercity.com costs about $40 to join, but you get a list of sitters and nannies in your zip code area to contact. The sitters sign up with the website and post their biographical information. They have to be over 17 with certain credentials.
We keep meaning to sign up but so far, we can cobble together babysitting among friends and family.
We found our nanny thru word-of-mouth, which is also a great way to find someone. Just ask all your friends and coworkers to get the word out about what you are looking for. We found our nanny from a friend working at Microsoft who was perusing Microsoft's parenting listserv and thought of us when she saw a posting for a nanny share that was perfect for us! You never know, just keep asking everyone for ideas....