Best resources for finding an apartment in Manhattan Valley, New York City?

Try craigslist in the "no fee" or "by owner" section and type Manhattan Valley in the search. If you can afford a broker (10% of a year's rent), then that's the best way to go. Unfortunately, brokers run the rental market in New York and it's hard (but not impossible) to get into one without them. Manhattan Valley should be a little easier, though, because it's not as in demand as other neighborhoods.