Went to a farm auction couple of weeks ago. These are often treasure troves for people like us who reuse, refurbish, rebuild. There was a three section spring-tooth harrow that they couldn't get a bid on for a while. I saw all that spring steel and thought of my blacksmithing area in my shop and realized I'd NEVER be able to buy that much spring steel affordably from a materials store. I bought the implement for $25 and trailered it home and put it in the treeline where it will live out its days supplying me with mild steel and spring steel for a host of projects or repair jobs.
Parts aren't always found in the form of a second "parts" version of the thing you want parts for. With a bit of imagination you can see things that can be fabricated out of other things. Thrift stores, garage sales, estate/farm auctions ... all are great sources where you can find your next part ... with a bit of modification.
No, I'm not going to find a watch sprocket in a two-bottom plow. But, there is a lot out there that can be repurposed for what you may be trying to do or need.