I've been looking around lately and of course, I can't upload every last Duke map to the workshop. I've found a few sites that may be of use to some uploaders:http://www.oocities.org/timessquare/5744/duke3d_graveyard.htm
All links appear to be functional, beware of the maps that require custom ART.http://www.oocities.org/timessquare/chasm/7733/archive1.html
Some interesting maps and descriptions.http://duke3d.freehosting.net/maps.htm
There appear to be some good ones here too.http://duke-zone.tripod.com/Links/id2.html
None of the links work here but, you can use them as a guide to try and find the maps from another source using the zip file name.http://dukeworld.duke4.net/classic%20dukeworld/maps/
This is pretty much one of the biggest databases of Duke maps. I wouldn't recommend uploading every single one but, you can use it to find maps that may not exist anywhere else anymore.http://dukerepository.com/
Probably the best source for quality maps. Make sure that you check the author and dates on some of the maps as some maps are recent and some authors are still active so you may want to contact them.
Make sure to follow Termit's publishing guide here: http://steamcommunity.com/sharedfiles/filedetails/?id=198993334
Also, try not to upload duplicates of maps on the workshop :)
Oh and if anything comes with a Midi file, just link me to it and I can quickly convert it to an OGG file for you to use with your upload.