Check large trash day. Lots of people will put fallen limbs and smaller stuff out at the curb so I'll go up if I can identify something I want and ask the owners if they're cool with me taking it, and then make sure I don't mess their drop off pile up.
I started when I needed aquarium wood and wanted to age my own, though that gets a lot trickier with determining whether they used pesticides or anything and people can get kinda standoffish about their plant care regimen being investigated 😂

Be mindful that they're not obligated to let you take it or may consider it a hassle just to deal with you so you may get more no than yes. Especially helpful if you can find their number to call as people are getting jumpy around others coming up to their doors for some stupid reason. (At least in the US)