While downloading BGM for personal use as a ringtone generally falls under "Fair Use" (as you are not redistributing the song for profit), there are a few things to keep in mind:
To ensure the highest audio quality and get the exact 30-second snippet you want, creating your own ringtone is the best route. Here is how you can do it legally: Sillunu Oru Kadhal Love Bgm Ringtone Download