Cheap is nice, free is better
If you wanna build something on the cheap (and have access to a truck), I have scrounged some NICE lumber for free...Cruise by any commercial glass shops in the area...They get large plate glass in frameworks, made of 2x4,2x6,2x8,2x10 even 2x12(as well a 1x4 and 1x6s,make good braces)...much of the stuff is just as good as you'd get from the local lumber store, and sometimes up 10-12 ft long....I used to carry a coupla wrecking bars, a 3 lb. hammer and a sledge around in my truck(they use ALOT of nails in these things), and a broom to clean up any mess I make...Most places are more than happy to give whatever you want, so they don't have to deal with dissasembly and removal...I've also picked stuff up on jobsites where they're installing windows in office buildings...I also have scrounged stuff from places that sell kitchen cabinets, same idea on a smaller scale...Sometimes get up to full sheets of plywood of varying thickness as well.
P.S. I stopped scrounging a while ago, as I now have 1/2 of the bottom of my barn (equivalent to a 2 car garage) FULL of "reclaimed" lumber. Built multiple benches and all kinds of other stuff out it. Working on framing in the upstairs of the barn, to make it weatherproof/heatable...All for FREE (yes, I'm a cheap SOB)