I believe TSE and Police Ordinance are the sources for the Advantage Arms kits. I got mine from PO and it ran ~$370 for the G21 IIRC...
EDIT: I'm an idiot sorry. I didn't notice you were shopping for one for the longslide...I don't know if the standard AA kit would still work. Might be worth giving JR/Redleg or Jo a call to ask...