There was an organization in Central Illinois that got first crack at the last days of old antique homes coming down. Most had fallen into disrepair or were just old farmhouses and so forth that were in the way of new development. Some had nice architectural elements like old light fixtures, fireplace mantels and so forth and salvage crews would grab these things for sale in the warehouse.
What caught my eye a time or two were gorgeous, century old, never (or minimally so) refinished hardwood floors. It took some labor to pull the flooring up carefully so as not to damage the T&G and denail it but it one case I found some beautiful cherry, milled in 1875 but never sanded after installation, that became new flooring for a kitchen and dining room renovation. I doubt I could have found such flooring new.
Some times the T&G on flooring bit the dust trying to get it up but some of the narrow oak I found made nice, straight nailed ceilings. I did one library home office for a client with it, some rich vintage wallpapers and nice recessed lighting.
Special turned elements ended up rescued and for sale at the warehouse but trim in end of life antique homes was usually free for the taking if you had salvage rights. The problem was finding enough space to lay it out to work on it. The oak and hardwood baseboards and so forth were often worth the effort.
I had to pay a little for some of it but recycled door hardware for a time. I sent it out for rebuilding, replating and so forth and ended up making a little money on most of it. So hang on to any of that sort of stuff. Old warehouse and barn hardware is usually in demand.
A newspaper publisher accumulated pallets at a pace hard for them to dispose of so they offered them to members of the public willing to haul them away. They tended to be broken as used so couldn't be returned for reuse or anything. I pulled more than a few apart, ran the planks through a planer to even them out, sanded them and stained them for walls of a basement rec room. It was untreated wood so scraps were safe to burn in the fireplace.
I would suggest a planer plus a stack of just about any flat lumber has possibilities for siding or paneling applications.
We had a great Habitat for Humanity ReStore in town too. Depending on how accounting creative you dared be, you could claim ReStore purchases as donations to Habitat---thereby getting things for free I guess. Made me squeemish so never went that extreme myself but did find some real bargains. And it felt good to be recycling materials and helping the Habitat cause.
Some contractor once left a huge amount of really nice, contemporary Italian ceramic tile that must have been ordered as part of a huge department store flooring project or something. I got enough to do two baths and an entry and paid next to nothing for it. The project budget could not have withstood tile of its kind had I not found it.
Same place had really nice track lighting fixtures a large department store was switching out. Fixtures were $1 and the bulbs $.50. I just had to buy track, wire connections, etc. It all went in a gallery space and I figured the savings to my client was $600 or so.
I used to see 5 gallon buckets of nice primers and paint (real paint store paint) but it did not last long and you had to be able to use institutional sorts of colors. I think ReStore got $10 for 5 gallon buckets of paint.
Anyhow, it was well worth breezing through the place now and then.
I don't know if landscape material falls into the same category as building materials but people seemed more than willing to share groundcovers and certain spreading windbreak types of plant materials. And of course one man's rock in the wrong place became a landscape design focal point just for the asking at times.
Repurposing things is always fun I think. The preservation people were not wild about seasonal runs on antique panel doors, but stripped, trimmed and refinished they do make lovely headboards.