Parts like that are pretty much all still available from DEERE.
I'm GUESSING you need the idler sheave #16 in this diagram, DEERE part number is AM38171, some $20 through the dealer.
If that's not what you need, you can go to www.jdparts.com and look it up for yourself, using either "New Parts Catalog" or "Previous Parts Catalog". I personally prefer the old version. To look up parts for the tiller, I believe it is a #31.