UT2004 is still available at GOG and Steam, unless you are shopping from a German IP. Due to legal issues the game is not even displayed in the search results.
You should plan a few minutes for the installation. Even from SSD with a reasonably up-to-date PC it takes a while. On older computers from DVD to disk it can take up to 45 minutes…
After installing, you should install the UT 2004 patch and the bonus pack. Then you have at least the standard maps. But since not everyone has the same understanding of “standard” maps, there is also a map pack that you can unpack with 7 Zip. The maps belong to
\UT2004\Maps. With the Unreal Anthology installation this is in an extra directory. For those of you who didn't install the game from the Anthology DVD, but maybe have it from the UT 2004 DVD, it might also need the crack. Then you can avoid the DVD while playing. Without a valid CD-Key nothing works anyways. UT2000 is also available at gog.com. But not anymore from IPs in Germany.
The Mobile Infanterie server is available on the standard port:
Ports 7777 and 7778 (UDP) must be forwarded for a UT 2004 server. As with UT 1, 7777 joins the server and 7778 joins the query. The server can then also be reached directly via the Internet.