Google for "german subtitles" and you'll find quite a few sites where you can download German subtitles for various movies as a seperate file. That may or may not be legal in your country, I don't know. Ask a lawyer if you really want to be sure.
Many video player programs can combine any video source (e.g. a DVD) with this subtitle file, even if the DVD itself hasn't German subtitles. So any video source with a German audio track is fine. Nearly all foreign movies are dubbed in Germany, so you shouldn't have any trouble finding a movie with German audio somewhere.