You have to prowl craigslist/ebay continuously and be able to get there immediately to score a deal. There are even stupendous deals in your Boro, Brooklyn, but for every underpriced bike there are 10 plus buyers. Know what you're looking at, don't ask a lot of questions, figure your tolls into cost, and have the cash ready. I like vintage steel.
Here are some recent scores in the NYC metro area.
1983 Trek 500 - Parsippany NJ via ebay, 3/2016. Although I am in Florida this week, I won, paid for it, and will pick up around Easter.
1981 Miyata 1000 - New Hyde Park, NY via craigslist, 12/2015. Somewhat cosmetically challenged, but about 25% of FMV.
1988 Centurion Ironman Master - Lincoln Park, NJ via ebay, 12/2015.
1987 Centurion Ironman Expert - Tappan, NY via craigslist, 10/2015.