I've been doing quite a bit of research while trying to get SC3K Unlimited to run on Win7 x64 from it's original CD.

There are still a few sites that host custom content out there. I'm more interested in attaining all the official files Maxis released, but "custom content" could be included for a few of these websites.

Here's what I've found so far regarding updates and such:

Widescreen Patch
https://www.gog.com/forum/simcity_series/simcity_3000_widescreen_patch_instructions

Enable Additional Music Tracks
https://www.gog.com/forum/simcity_series/sc3k_enabling_the_three_missing_music_tracks_howto_guide

There's an mp3 soundtrack linked in this post. I believe there's also a tool to convert WAV files to XA files which are used by SC3K. It would be possible to use an audio converter to convert the mp3 files to WAV format and then to XA for additional ('unreleased' hehe) tracks to use in-game.



Regarding Official and/or Custom Content:

Note: I'm not sure what is included in the GOG version of SC3KU, but I've read in various places that you may need to upgrade the BAT (Building Architect Tool) system to a Landmark

(Maxis Official) Landmarks Package
https://www.gog.com/forum/simcity_series/simcity_3000_unlimited_landmarks

CityBuilders
(For all things SimCity. Includes Maxis Official Files from the old Downloads Exchange! Requires registration to download)
https://city-builders.info

Here's a log of the creation of CityBuilders. Might be useful to look at old posts or links left in people's signature areas.
http://www.sc4devotion.com/forums/index.php?topic=15853.0

SimCity 3000 Resource Center (A bit dated, but hosts some interesting files)
http://www.sc3000.com/files/



Here's the link (via The Wayback Machine, courtesy of Archive.org) to the official Maxis SimCity Exchange, though the files available for download are inaccessible due to the fact that you need to be a registered user in order to download any content there. But for old time's sake! (And for future [/past] reference)...
http://web.archive.org/web/*/http://simcity3000unlimited.ea.com/

I'll come back and update this post if I find anything else out there.
CityBuilders is probably your best bet for any official or custom content.

Thanks for mentioning my site (city-builders.info), in addition to the links you have given in your post, I've been collaborating with Simtropolis (its a GOG affiliate), to revive SimCity 3000 on it.

The download section is split into two parts the official maxis updates that are on my site, the Simtropolis staff are still copying the files across and the custom content section, I know the site would welcome any SC3 players and creators.

-catty

Sorry it won't allow me to post links

Link to the CityBuilders Website - www.city-builders.info is now dead, as life issues meant I closed the site down late year.

All the files and documentation were moved over to the Simtropolis site

Custom Content and cities are here: www.community.simtropolis.com/files/category/41-simcity-3000-files/

Documents, etc are now in the Simtropolis Omnibus here: www.community.simtropolis.com/omnibus/

Anything else that didn't fit anywhere else from my site went over to here: www.community.simtropolis.com/clubs/26-city-builders-website/

as for the SimCity 3000 Resource Center which is here: www.sc3000.com

Its also hosted by the Simtropolis website unfortunately age is catching up with it and I have volunteered to copy over all the SimCity 3000 articles over to the Simtropolis Omnibus ... the Omnibus will be having a make-over at a later date in the meantime you can check this document to see if something has been copied over and were it is.

www.community.simtropolis.com/omnibus/simcity-4/reference/omnibus-sitemap-draft-only-r571/

-catty