J.M.
In my search for babysitters I came across a nanny service that looked promising although I never used them myself-- but may be worth checking out. Their website is http://www.premiernannyservices.com/ and they are based out of Bucks Country (which is why I ended up not using them myself).
I ended up using craig's list. Have you heard of it? The local craig's list for Philadelphia is a place to post ads, sell things, etc. online. You can try that also. It is http://philadelphia.craigslist.org/. There is a special section entitled "childcare". I've found some really great babysitters through that free service.
Best of luck.