Property shark is one of the best sources, or you can ask an agent who could get more information for you. I'm guessing that you want that information to give you an advantage when you negotiate? Like Paul answered it's best to look at the current market conditions, what someone purchased something 5 years ago, means nothing, if they paid $500K and you want to offer $500K now, well you know how that will play out.
Now on the other hand if you want to find out how much a family member or a friend purchased a place for because they are not telling you, thats a whole other story.
Speaking of apartments I'm selling a Condo on 10th Ave and 51st in BoroughPark/Sunset Park.
Let me know how it works out with getting the price Chris.