Scrounging is built into me I can’t walk past a skip or pile of rubbish without seeing if there is something that I can scavenge or scrounge. Piles of rubbish often have a few gems in them. A broken appliance can be stripped for wiring, motors, interesting metal components for decoration.
There are always free sites like Freegle or FreeCycle where you can ask for specific things or browse what people are offering.
If you are a student or work with children (and in Bristol) then you can check out the Scrap Store (its awesome!) or you might be lucky enough to have a tip shop.
If you are looking for wood for a project and can’t scavenge what you need have a look out for a wood salvage organisation, such as the here you can buy wood cheaply and they often have offcuts that are available even more cheaply or by donation.
If you are looking for metal components or frame work then keep an eye out for old PC’s, components and appliances. You can always visit engineering workshops and buy offcuts or scraps from them for much cheaper than you would be able to get material from a hardware store. Old PC’s are great for robot building as you can strip CD/DVD/Blu-Ray drives for the motors and LED’s. The frames of the PC cases make good boxes for prototyping as long as you have big enough motors.
I know in Bristol they do not allow you to take things from the tip but I’m sure that in some areas or countries they are more than happy to let you take anything that you could use, but here’s the health and safety caveat – The reason that you are not allowed to scrounge in a tip is the danger of you getting squashed by machinery, spiked, cut, broken, infected or any other manner of hideous things. If that happened then someone somewhere would sue and make it prohibitively expensive to do anything, it’s much easier to blanket ban scrounging.
One occasional exception to the rule is building sites, if you speak to foreman or site manager they may be willing to let you remove things from a skip. It could reduce the cost for them of disposing of their rubbish. If you are going to do this then be polite, don’t get in the way, be tidy and if they say no respect their wishes and walk away, there are always other sites!