restaurants, pizza places especially, Papa Johns pizza at least used to get their sauce in #10 cans
Recycle bins, or recycle collection points
School or church cafeterias, soup kitchen
When dealing with food trash, take a hooked stick to poke with, and wear gloves, those are gross trash bins.
My dumpster pack has gloves, paper towel, bags to put stuff in, water to wash with, hand sanitizer, band aids etc.
I had a cop tell me "You ARE doing this carefully, aren't you?" Yup. He was worried I was either starving or being a problem. I told him all about the things I was getting out of that one, how I'm using them to redo my kitchen... He decided I was eccentric and gave up :)