Here is another location on Long Island, out in a State Park in Montauk, that has fantastic Filiformis!! They grow in a wet, sandy swath out in the sun and circle back into the bog and grow bigger and greener with Intermedia.
There's another location nearby in another State Park with much bigger Filiformis and loads of them! These grow among grasses and other competing vegetation along with loads of Intermedia and tons and tons of Rotundifolia.
There is another location of Filiformis on LI that I know of, but there's very few of them there... the so-called Coastal-Plains Ponds which are badly overgrown with no Plains whatsoever. The informational sign there says that they are common. Maybe they were at one time... land is managed by the Nature Conservancy too.
These photos here are from the most stunning location in the open, sunny and sandy location in Montauk.