There are two common types of Acoustically Transparent (AT) screens: perforated and woven.
Stewart is the perforated style I see used most often; they aren't cheap.
Woven, there are two companies that are used most often - SMX and Seymour A/V. SMX used to sell the material separately, but not anymore; Seymour still sells the material separately. A premade screen from either will run somewhere in the range of $1000-$2000. I wanted something cheaper, so I bought a frame from Jamestown Home Theaters, and the Seymour DIY AT material, and was able to come away with a big AT screen for several hundred less than I could have otherwise. Others build their own frames and add the AT material.