Where and how you can get building material for almost everything you plan to build? Look around, there are goldmines everywhere, skips full of 100 year old pine, ripped out of some apartment that is being remodelled, especially if you live in a big city. I find at least two skips full of nice wood every time I go shopping or drive to work, not exaggerating here, there are flats being remodelled everywhere. And the construction team on site will be happy to get rid of some of the skip content for free- just ask them, see what they say.
There are also places where you can buy some amazing used wood pieces, like fat beams, railway sleepers and so on for dirt cheap- and they can even cut them to order saving you time and effort- I found one close to my shop, amazing stuff and cool people-worth giving a try.
Things like nails, screws, stain and hardware you will have to buy. I do not think recycling nails is something that you would like to do. But you can shop smart, to stay on the budget and still get what you need. When you go to B&Q go straight to the sample and scraps section. It is where the employees cut wood to order for some people, or where you can find paint that was returned, and so on. You can score some amazing deals for literally peanuts and thus make your project this much valuable.
After a while you will train your brain to pick up clues and tips, you will start noticing things you have never before paid any attention to, just like being young and afraid of becoming a father-all of a sudden you see pregnant women everywhere:)