ROSEBRAND Theatrical Fabrics in New York City is who I use. (www.rosebrand.com)@ (800) 223-1624.
My personal favorite — although expensive — is Black SHARKSTOOTH SCRIM, which is available in 35′ widths and sold by the yard.
I use the much cheaper black BOBBINETTE everywhere …from cutting down exterior light coming through windows to in front of light boxes to reduce glare (and fluoro intensity). It’s available in 10′ and 30′ widths, sold by the yard.
When I’m on a really tight budget I’ll even use black TEXTILENE — but it only comes in 54″ and 98″ widths.
DS