E.B.
I've had luck finding someone on www.sittercity.com
You can post exactly who you are looking for and how much you can pay, etc. and people apply for the job. From my recent experience there I can tell you that there are A LOT of young women out there looking for summer work. You can subscribe for a month for about $35. Also, check the Member Perks section of Mamasource for a discount code to use.
Good luck!