I keep my eye on kijiji and keep an ad on there for hollow core doors , I got 4 last week and the lady said she'll have another 3 for me soon. I only look in the free section otherwise I may be temped to spend!! lol
I look for "recycled" 2x4s for framing, and then I get many, many sheets of 7/16" OSB, also known as sheathing. It's strong, sturdy, and really cheap -- about $5 a sheet.