For some subtitle translation, pre-made subtitles may be a useful resource particularly if the video is a well-known or commercial work. For example, if you are including a scene from an American documentary in a video, there are resources to search for existing subtitles in a given language. However, outside of well-known video and cinema, pre-created subtitle resources are few and open source resources are even fewer. When they do exist, they come in the form of open source corpora and translation memories. Both are a type of repository for parallel translated language phrases and segments. Subtitles are then able to be translated with a search and find technique. This can be an especially useful tool for translating idiomatic expressions and common word strings.
There are a few issues that come up when searching for subtitles. For cinematic films, for example, there are almost invariably many different versions of the film. One can imagine that any extra scene, extended title sequence or formatting change can alter the timing of subtitles onscreen which many times renders subtitles useless. Therefore, it is important to find subtitles that are accurate for the audio of the particular film version. There are tools like the open source Sub Downloader (http://www.subdownloader.net/) that help with this problem by matching subtitle sets to specific film versions. Another issue that comes up is the file format of the subtitle file itself. There are different formats for different types of video as well as different types of physical media (HD, DVD, Blu Ray etc.) which affect the selection of subtitles for a given piece of film. In short, details about the film and audio change the availabilty of subtitle resources.