And I recently learned Blackhawk makes their serpa holster for the 5900 series:
BLACKHAWK SERPA CQC #10, RH, S&W 5900/4000/9/40 SERIES, LOOP/PADDLE HOLSTER.
BLACKHAWK SERPA CQC #10, RH, S&W 5900/4000/9/40 SERIES, LOOP/PADDLE HOLSTER.