Replacement Fans

The hot end and motherboard fans that come stock with the D6 are very poor quality and loud. When replacing the fans, you should look for 24v fans around 40x40x10mm

Here's a list of recommended direct replacements:

NTE Electronics 77-4010D24 High Speed Fan, Ball Bearing, Thermal Plastic Wire Lead, 40 mm L x 40 mm H x 10 mm T, 24 VDC, 7000 rpm, 7.73 CFM

414F ebm-papst, Inc. Axial Fans, DC INPUT SQUARE Series

WINSINN 4010 24V DC Cooling Fan 40mm 40x40x10mm for DIY 3D Printer Extruder Hotend Makerbot MK7 MK8 CPU Chip Arduino - 2Pin 0.1A 2.4W 6200+-10% RPM (Pack of 5Pcs)

If you know how to wire fans in series and know the risks of running PWM fans in series, here are some 12v choices:

Noctua A-Series Cooling Fan Blades with AAO Frame, SSO2 Bearing (NF-A4x10 FLX)