Depending on what you are looking for you have some options. No matter what you choose think about the cost. You can save a lot of money by following one of the methods below but remember your time is worth something to.

General post type material – look for houses in the spring and summer who are redoing their fence. You can always take the old stuff off of their hands and they will be grateful because they won’t have to haul and pay to trash it. Try any one of the neighborhood apps for this. Or just drive around.
Not so great but still a way – go to Lowes or Home Depot and look for their scrap area. Ask first! Sometimes you can find gems in there.

Speaking of big box stores – you can get a discount on wood it they consider it more-or-less ruined. Basically something that’s either really bent or looks like a Wookie took a bite out of it. They don’t have to do this but it doesn’t hurt to ask.
Flooring – go to any flooring store and ask what they have in the back waiting to be destroyed. You will be surprised at what you can get there.

Pallets – You either love them or hate them. Find ’em in random spots. I like to use them for walls or floors. I don’t take them apart very much as I can only Dremmel out so many nails.