As of June 2015, you can buy accessories for the Mossberg 930 SPX online from GG&G, MidwayUSA, and Harris Tactical. These accessories include aftermarket stocks, sights, scope mounts, magazine tube extensions, springs, safeties, shell carriers, flashlight mounts, sling mounts, bolt handles, and rail systems.
GG&G manufactures a variety of accessories for the Mossberg 930 SPX and sells directly to the consumer through their website. MidwayUSA and Harris Tactical sell accessories from various manufacturers. As of June 2015, Harris Tactical is selling a full quad rail system for Mossberg 930 shotguns for $150, and Mesa Tactical Urbino stocks are available from MidwayUSA for $144.99 to $174.99.