I find it EVERYWHERE! I go to thrift shops, yard sales,
junk yards, and pick up things beside the road!
If it is metal, and not rusty, there are ways to make it rust.
Google--or pinterest--"how to rust junk"
There are even ways to make wood look like rusty metal.
I sometimes find more than I can use-LOL

You might try a barn sale too! I found a ton of stuff in a PA barn sale last fall. Also yardsales are good place and thrift stores.

Put a cup of salt in spray bottle and fill with water...shake good and mist your metal. Do a couple times a week and it will rust fairly fast.

In the trash or at estate sales. Head for the garage first! I love rusty old junk.

Sales (yard, garage, barn), trash and the neighbors (old tool heads that the handles rotted off of most recently). I mostly avoid estate sales here since most of them are over priced, but I've had luck at some private ones (I got a bunch of old crates and things at one private estate sale at a farm). Garbage picking is hard now for anything metal, since so many people are "scrapping". You can also try freecycle or craigslist.