I got them from New World Aquarium. I combed about 7 or 8 hardware and plumbing stores in Manhattan (Midtown, Village and UES) and the Home Depots (Chelsea, Midtown, Glendale) for it, and couldn't find it.
They were $2.99 each. It hurt, but it's the price I pay for not wanting to buy online.
They offered me an alternative for my return tubing (I forget what it's called, but it's basically PVC tubing with a swivel head that you connect to the vinyl return line), but it cost $39.99, and what I wanted to make only needed 5 pieces of barbs/adaptors which cost $16.
How I wish there was a more comprehensive plumbing supply store in Manhattan.