Where to buy open cell foam in NYC(?), and question about closed cell

I would say RagShop, but alas.

Avoid craft stores like Michaels. They have NOTHING fabric related.

Just thinking about old couch seat cushions  Open celled foam I believe and really cheap

I actually used to go to Wal Mart into what used to be their fabric section and picked up Sofa/Couch Foam.  It's a little expensive, but it will do the trick.  Egg crate foam also works, but also can get costly depending on the type.  Plus you have the added detriment of it not being square for arrowheads.  I'm not sure what's available in your area, but you may want to look around to the textile stores in the fabric district of NYC that Paul suggested.  I know you can get some of the couch foam there.  In fact, that was where I got my first set of it for boffer weapons back in the day.  If not, i can bring a sheet or two to the next event.  It won't do you good right now, but you'll have it for later if you can hold off until then.

the fabric district around 7th and 38th aka the fashion district most likely will not have foam. There are very very very few home fashion  stores in that area. You'd have to look further uptown in the 50s range, for that.

- Jacqui

As a Michaels employee, I can truthfully say that Tom has the right of it.  We don't carry fabric.  I was in a Joanne Fabric yesterday, and they do sell open cell foam by the yard, but it's a mite expensive.