Here are the main sites where you can download DMGF original simfiles:
GDAmania: Multilingual site. Currently has a complete set of DM simfiles and all CS released keysounded DMGF simfiles. As sherl0k mentioned, you need to register in order to download.
Jinnai's website: Japanese site. This is where a lot of Japanese GDA makers post their GDAs for DL. In most cases you have to comment before you can download. This site is occasionally blocked to people living outside Japan.
Neosolo: Korean site. You need special authorization to download their GDAs. Try emailing the webmaster.
There are a lot of other small Japanese sites, but those come and go so I won't post them here. Just search in Google or look at the links sections on GDA sites. A lot of these links can be found in the links section of GDAmania.
Here are the main sites where you can download DMGF original simfiles:
Jinnai's website: Japanese site. Click on the 3rd star (代理). This is where a lot of Japanese GDA makers post their GDAs for DL. In most cases you have to comment before you can download.