Best resources for finding an apartment in Upper East Side, New York City?

Craigslist has the most comprehensive listings out of any place in the city believe it or not. You can also try a few of the brokerages' websites. Bond and Citihabitats are two of the biggest and all of their listings are on the site. A broker charges 10 percent of a year's rent which is insane, but, unfortunately, they are pretty hard to avoid in Manhattan.