Locknut for Idler Pully

[JIMSVX]

This weekend we traveled to Indy (500 mile round trip) and when we started to head home I noticed I had no power steering and no charging from the alternator. The idler pulley assy had popped off the tensioner bracket bolt and was hanging on the belt. The locking nut was gone, nowhere to be found.

I headed over to Ace Hardware with the bolt and picked up a $0.45 nut with a nylon locking insert. I didn't want that thing to come off again on the way home, and it didn't. In less than an hour we were on our way home. I'm happy to say that in the year that I have owned the car I did not touch that nut. Whoever changed that belt last was the culprit who didn't tighten that nut.

I want to get the correct replacement part but I can't find the part number on any of the Subaru dealers' websites. I also can't find a picture of the correct nut. This is the best thing I can come up with.

And, this is what I used as a temporary fix to get us home.

Can anyone help me with a picture and/or part number (or even a replacement nut)?

Thanks!