The only thing I know is only 100-500 (depending on who you ask) of these kits were made in 2003. PWD/TOP discontinued production of these. The. Most of them sold in Asia and to Europe. Because most US customers wanted the Smokey's M82 which came out the exact same time. There's something about the US market that they shun the bullpup design for some reason. With an upgraded APS2 the M99 + APS2 costs were realtivley the same as the M82. The PWD M99 was more accurate then the due to using 6mm over 8mm bb's.
Also Gamepod.com claims to have a Smokey's 8mm version of the M99 for Special Order. But I dunno the validaty of the claim. I've only seen Smokey's advertise the M82's http://www.gamepod.com/store_037.htm
If you do happen to find a PWD kit, make sure you get a Maruzen APS2 EX for the internals. It's the only APS2 with adjustable hopup.