Where is the grabber file tv_grab_sn located?

It's in /usr/local/bin.

However, I guess it isn't working, as the source (dagenstv) has apparently blocked it.
What you might do though is change the user agent in the script to look like internet explorer for example. This is an unverified solution I got from a person in IRC (unverified, meaning *I* haven't checked it, but the guy said it worked).

where should i place the modified file?