SitterCity.com is pretty good and there's one other other there (name eludes me right now)....but even with that one you have absolutely NO safeguards and I found out you have no protection from by the company if a sitter goes 'South' on you.
Can find someone really close to where you live fairly easily. But definitely HIGHLY recommend going through their entire interview/screen process including the background check. AND USE THE SITTER CITY EMAIL ADRESS that is given (not your own). Recently had a bad experience with some girls posing as 17 year olds (have to be this old to even be able to post at this site) and discovered when we got to the background check time...that they actually were only 15 & 16 and didn't want to go through the check to reveal the age fact to the site. They're considered minors, so can't do one on them! Very sad as the one gal had sat a few times and was really nice. And, its not that younger sitters are bad at all. But, there's something to be said for honesty and entrusting someone with your little ones. Rules need to be followed and obeyed...and right off the bat they were being dishonest.
There are alot of great people on the site though...but don't let your guard down as you do need to truly do ALL OF YOUR own leg work up-front.
Best of luck!
PS: Better options: Putting up posting at your local church where you actual know the people, local colleges where there's a better maturity level (educaiton depts) is another good source. And have heard that 'Nannies From the Heartland' is especially good...costly, but they do alot of legwork and have way more accountability for their babysitters that are sent out there.