If you have a dvd with you and you want to rip subtitles from that for just 1 language, say 'english', then if you have 'K-Lite Codec' installed you can simply use 'VobSubStrip'.
Steps:
1. Simply copy the .idx and .vob file from your dvd to your hdd.
2. Open
VobSubStrip and load the .idx file in it.
3. Select the language or languages you want to rip.
4. Select an output directory and click process.
5. A new .idx and .sub file will be created with the selected languages.
Mind you, an avi file works with both .idx+sub or .srt files on both dvd and on divx dvd players. So converting from .idx+.sub to .srt is not really necessary. Unless ofcourse you don't like the fonts in it.
In that case try Subrip. Download it from here:
Subrip
If don't have a source or just need subtitles look here:
subtitles - Google Search
The first 6 links should help.