Several filter makers like Marumi, Tiffen, Lee Filter and Cokin make their own filter wallets. In addition, Adorama, Tamrac, Massa and Lowepro make wallets as well. There are also several OEM brands about, try here.

Adorama, view here.

Arri 6×6″ Filter Pouch here. Other models here.

Clik Elite 4×6″ here.

Kinesis F169 — Large Grad Filter Pouch here. Other bags here.

Lee Filter pouch, here.

Lowepro filter pouch, here.

Mindshift pouches here.

Terrascape pouches, classic, small one, and onesixfive.