Interesting BG Tracer Unit that is in stock at EB Airsoft right now. This BG BD0478 ILLUMINATION Silencer for Glow BB is made to fit all types of gas-powered airsoft guns  and even shotgun series. It will also work for certain types of AEGs so check the image below which AEGs can't be fitted with it. It is made of aluminium with a 14mm negative thread.

It is powered by 4 Triple A batteries and the package comes with a sample set of tracer BBs. Price is US$34.95, shipping included.