I'm hoping there is an easy fix for this, so if anyone knows it, your assistance will be very appreciated.

I can download music and small video files just fine. When I try to download large video files (around 500 MB and up) it just says connecting for a long time and then it says "Needs More Sources". I tried many different video files and many different formats (.mp4, .mpg, .avi). Does anyone know why I am only getting this problem when I try to download large video files?

You might be trying to download fake files. These will sit connecting for ages & then get stuck needing sources forever. If you're trying to download movies which are still in the cinema & haven't been released on DVD yet, there's a good chance that most files showing up in search results will be fake.

Always check a movie with Bitzi before download. If other users have rated the file, you'll get an idea of it's quality & content (can help you avoid misnamed files). If a file has heaps of sources but no Bitzi rating, chances are it's a fake. There's a link to Bitzi info in this thread.
Fake files showing up in search results

I looked up a lot of them in Bitzi but none of them are in the database. I'm assuming there are a lot of fake files. This is an older movie though, so there should at least be a few authentic versions.

Yep, there are fakes of older movies as well. Try using the All Types search & look for movie title - format (eg blahblah - avi). You could also try varying your search terms to look for actors' or director's name. Sometimes you just have to keep trying at different times of day...as users log on & off, files available change.

The LW Beta versions have torrent support. The latest Beta is 4.13.4 & you can download it from the usual LW download page, or from your personal download link if you have Pro