The Snow Wolf 500-round high-capacity magazine for the M82A1 AEG is available from several online airsoft retailers. This magazine is designed specifically for the electric (AEG) version of the rifle and is not compatible with spring-powered models. 
 
Key Features
Capacity: This is a high-capacity (hi-cap) magazine designed to hold approximately 500 rounds of 6mm plastic BBs.
Mechanism: It is a wheel-wound magazine, where you wind a wheel on the baseplate to compress the spring and feed BBs into the rifle.
Compatibility: The magazine is specifically for the Snow Wolf M82A1 AEG (electric airsoft gun) and M107A1 AEGs. It is not compatible with the spring-powered versions of the M82 or M107.
Construction: The outer shell is made of metal to replicate the feel of the real firearm's magazine, while the internal components are made of polymer to keep the weight manageable.