You can find addons in the HD wiki, here are some extra ones:

====HIDEOUS DESTRUCTOR BOOT CAMP WAD=====
https://forum.zdoom.org/viewtopic.php?f=42&t=65873

======Hideous Destructor addons=======

ZikShadow's graphical addons: Bunch of reskins for your guns
https://forum.zdoom.org/viewtopic.php?f=19&t=53251&start=0

PotetoBloke's graphical addons: More reskins
https://forum.zdoom.org/viewtopic.php?f=43&t=60179

Accensus HD Graphical addon: adds prettier sights, a new revolver and a new shotgun, compatible with the addons above
https://drive.google.com/file/d/1OdOpVqDAr1i7Uz49Dr3FxUNY4MDrYcgv/view

Tartlman's HD addons: more skins and a zerk mod
https://forum.zdoom.org/viewtopic.php?f=43&t=66091

===Compatible with HD====
Immerse:
https://forum.zdoom.org/viewtopic.php?f=43&t=61915

Droplets:
https://forum.zdoom.org/viewtopic.php?t=46509

Nashgore: (REQUIRES AN EXTRA TEXT FILE TO BE LOADED ALONGSIDE NASHGORE)
https://forum.zdoom.org/viewtopic.php?t=62641
Extra file: (taken from the HD discord)
https://mega.nz/#!DnQ0DIRC!USJYW7iA6a1_Od_1jB7kcLQIXV3Z1d0iiO38gVGyx2A

Steve's Flashlight mod:
https://forum.zdoom.org/viewtopic.php?t=59429