1,800 Clams Needed!
I'm curious about this. Are you planning to do the clambake in NYC? With an actual open pit fire to steam them in? If so, you might get a call from the fire dept. 1800 clams for 100 people would require a pretty big pit and a fire that size would attract attention in the city. If its outside NYC never mind. Best place to go is to the Fulton Fish market and check things in advance with one of the vendors. That's a lot of clams. I think depending on the size, there's about 150 clams a bushel so you are going to have to haul 12-15 bushels. Get your old pick up truck to Hunts point..
I'd try asking sea food restaurants who they recommend and what tips they've have for making this kind of purchase.
I'd also imagine that you'd pay less if you bought them from someone in NJ and transported them to NYC yourself (assuming you have inexpensive access to a vehicle).
Braun's Seafood in Southold or Cor-J in Hampron Bays. I know they both deliver to Manhattan. The wholesale fish market moved from South St to the Bronx several years ago. For that quantity there should be a fish monger who will help you.
Otherwise, a local, dedicated sea food market should be happy to be a middle man in the transaction.