Seeing this is an old game some of the download sites may disappear over time. I will link some alternative sites here.

Live sites

MetaMod and AMX Mod X 

Full installers and Mod installs https://www.amxmodx.org/downloads.php

Developer builds (latest branch) https://www.amxmodx.org/downloads-new.php

Day of defeat plugins - https://www.amxmodx.org/compiler.php?mod=3&cat=0&plugin=&author=&go=search

AMX Mod X forums - https://forums.alliedmods.net/forumdisplay.php?f=3

 

AvaMods

https://www.avamods.com/news.php

Plugins... https://www.avamods.com/download.php?list.3

All downloads... https://www.avamods.com/download.php

 

Dead sites

dodplugins.net 

(link via Internet Archive)

This site and their forums is gone and used to be a great place to pick up server plugins for dod.

Fortunately, the plugins exist (in their hundreds) on a GitHub page....

https://github.com/fysiks1/dodpluginsnet_plugins_backup/tree/master

 

Day of Defeat Custom Content Forums Archive

This used to be where you placed a problem and learned about all things dod.

Steam... foolishly, shut this down in 2008. The new Steam forums are complete shit, no where near as good as this was.

You can still view the forum content as "wunderboy" (Jed - Neil Jedrzejewski. ) backed it up.

The Archive - Day of Defeat Custom Content Forums Archive

https://web.archive.org/web/20130415044403/https://dodarchive.wunderboy.org/

It's slow but enjoyable.