I don't know if this would now be downloadable. I have tried to purchase it somewhere but I don't think it is available. Duke Assault was an unoffical mission pack realeased for Duke Nukem with 1500 missions. Would love to get my hands on this. Anyone know where it is available?

I think it's unlikely. Duke Assault was originally published by WizardWorks Software, which shut down in 2004. However, it was owned by GT (now Atari). While I doubt that it would ever be republished, perhaps GOG can work something out with Atari if enough interest is shown.