At least at the Union Square greenmarket in Manhattan, the apple stand near the arch almost always has Greenings. They're definitely an heirloom style -- dull green with brown speckles, oddly shaped, hard as a rock and a good flavor once you get past the woody texture.
For a substitute, get something that has medium sweetness and sourness. Maybe an Arkansas Black Twig, which has the same hard texture.
Just looked through the paperwork on the apples I bought on Sunday from Scott Farm and sure enough - Greening's Rhode Island was one of the apple types I bought. Its a baking apple. I'm pretty sure they have mail order as well.
Here's the contact info:
707 Kipling Road
Dummerston, VT 05301