I'm making sort of a custom Druidism mod, and an option in it is to create a Netch on my home island that can transport me to any Netch breeding ground in the game... Unfortunately, I don't know where many of the breeding grounds are. Does anyone know the general location of any breeding grounds?

The only one mentioned in game is north of Gnaar Mok. As a part of http://www.uesp.net/wiki/Morrowind:Breeding_Netch

But I think anywhere in the Ascadian Isles, and most of the Bitter Coast is suitable for them.

http://www.uesp.net/wiki/Morrowind:Dren_Plantation has breeder's as well. There was also another Netch ranch somewhere in the Ascadian Isles but I cannot remember which house it was.