Construction sites (especially remodels!) are great places to rummage for wood scraps that you won't have to worry about making firewood out of. Those big industrial bins out front (we call 'em "skips" here in the US, for some reason) where they load all the trash? Find a foreman and ask if you can grab a board or two, usually they'll get someone to dive in and start handing boards out to you!
Also, old pallets can be good, if they're in decent enough shape. You can either pry the slats off, or use a metal saw, slipping the blade in between the boards to cut the nails. Then put a Phillips screwdriver on the newly-cut nail shaft (easy to find, it'll be nice and shiny), give it a tap, and that'll lift the head so you can get a claw hammer under it without gouging up the wood too badly. A little bit of effort, and boom, easy scrap wood for playing around, getting to know your saws better, and honing your skills.