Re-using wood from disassembled projects, temporary installations or even torn down buildings is a great way to reduce waste and can often give your products a very cool "weathered" look.
Also, it's often free! You may find materials in dumpsters e.g. at construction sites or stores but always ask whether you can take it!
Be especially careful when working with pre-treated wood or unknown species: dust or fumes may be toxic! Try to find out what you're dealing with and wear a suitable dust mask to be safe.
Pallets are usually treated against insects and funghi which may also have involved harmful chemicals - see if you can find the branding: for instance, "HT" (heat treated) should be save while "MB" stands for the treatment with methyl bromide which has long been banned in the EU! There's a nice breakdown on this at 1001Pallets.com if you are interested.
- Media Markt Kirchheim (ask at the delivery bay, they throw away tons of the cheap, deposit-free pallets)
- construction sites
- where else have you had luck?