Like a great vintage suit, finding a used/secondhand board takes a little digging, a lot of patience. But there are great finds out there if you're willing to spend some time trawling the internet and keeping your eyes open. Here are a few places to start your search:
Board swaps: These are great places to pick up a new-to-you stick for a really decent price. You also get to speak to the person who's been riding it for the past however long and get a few inside tips on how the board handles. There's a swap coming up June 7-8 at Sundown Surf and Ski. See their website for details.
NY/NJ Craigslist: In my experience, it's hard to find longboards on here--mostly there are lots of people trying to sell subpar boards for way more than they’re worth. Still, CL is usually the first or second stop in the search for a board on a budget and you may just find what you're after.
Ebay: look for boards sold in the NY/NJ area. Shipping is expensive on surfboards, so you’ll want to save yourself a headache by buying locally or trying to see if they’ll ship via Amtrak. Same quality/price issues as Craigslist, but deals can be found.
San Diego/LA Craigslist: Longboards galore, usually under $300. You can either take a trip to CA or communicate with the seller and ask them to put the board on an Amtrak to be picked up at Penn Station for a mere $50 (takes a little while to get there, but worth it for the great price).
Surfshops: East Coast shops don’t have used board racks to the extent the shops on the West Coast do, but they do exist. Longboards go like hotcakes. Pilgrim Surf + Supply, Sundown Surf and Ski, Maritime Surf, and Unsound Surf Shop all have used board racks. I’d call ahead to see about the stock before making a trip.
Swaylocks: At this online surfboard shop and forum you can search for boards by length and even post for surfboards wanted. Like Ebay and CL, you’ll want to look for boards being sold in the NY/NJ area or else figure out a way to get them shipped out here for as little as you can.