Check with Numrich.
Is it likely that a box magazine made to accept 3-1/2" shells will fit or be too large for a receiver with an opening designed to accomodate a 3" shot shell magazine. Measure the opening in your receiver.
Try calling the US dealer located in Florida to check on the availability of magzines for the IZh-81.
Or this outfit, which may have them for $80. http://www.gunbroker.com/Auction/Vie...Item=281833304