Speaker fabric can be found at Joann's in the upholstery dept. Ask them where they have it in the store. They might be out of it. I have also found it at Radio Shack. (These two are the generic black stretchy stuff)
Now we get to the cadillac versions - Guilford of Maine www.guilfordofmaine.com Their FR701 is acoustically transparent as well as other styles in their line. Many colors to choose from. Although you can get samples from this site, you need to find a source to purchase it. If there is an office "cubicle" manufacturer in your area, you can sometimes buy it from them. Otherwise, you can go to www.silentsource.com
Wendell Fabrics www.wendellfabrics.com Many different styles of acoustically transparent fabric to choose from. I believe you can directly purchase from them, too.
In addition to Pam's nice post: The Joann's near me stocks it on the same wall as the "pleather" (plastic leather) material/upholstery; very back of the store, far right corner. I've been in two different Joanns that had the same layout, so maybe that'll help. They refer to it as "Black Grille Cloth" (I do too); $6.99/yd at the store near me.
Matt, RS, PE and your local fabric store have limited cloth colors, (but they are cheap). Acoustical Solutions has over 75 colors. They have different types of patterns to choose as well. You can even get cloth for vintage speakers. They also make acoustic wall foam to splice perfectly with any grille cloth colors you choose. They send two free sheets with around 20 color samples per sheet.
Pam, their cloth cost more per linear yard than any I have bought from. But they have a huge selection and the quality of the fabric is second-to-none. I wish Parts Express would get more colors than four. They are more reasonable. Cheers, Scott