Used pallets can be a good source of wood. Where I used to work, we saved some that were a size that we would use again, but the rest were put out for the Pallet Scavengers. Unless they were in really bad shape, they disappeared.
Get the word out to neighbors and friends too. Our neighbor was GLAD to get rid of a metal pipe frame for a green house that was left from the former owner. It's the frame of my run now.
Hi! You live near me, and there is this cool hardware store that salvages materials and sells them for less. You could try that! -> https://www.seconduse.com/ . I am also in search of cheap materials, so if anyone has found any great ones, let me know! Good luck to the OP!