Hey Chris.

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.

go to propertyshark.com

it's free up to 6 searches per day, and will give you more info about each bulding than you could ever dream of.

It's very simple and you can do it on line. You need to find out the block and lot number of the building, or maybe you might find it under seller's name. Acris is not very user friendly but you will get the info. If you are not able to find it like that, go to the department of buildings, they have it, it's just a bit more work.

http://www.nyc.gov/html/dof/html/jump/acris.shtml

You can check with City Hall and they might have the records. However, you should base your offer price on todays market and what similar properties are selling for. Some stocks sold for pennies years back, but now cost much, much more. Market value today should be your main focus.