I like to build unconventional bikes from discarded material. Early in my adventures seeking out parts on the cheap I would go to yard sales and flea markets to buy old bikes. This worked but the costs added up and it took a lot of time.
Then a bike-riding friend recommended I contact a group in town called BikeAthens. It turns out they were a source of endless parts and advice. You probably can find a similar source in your town.
Step 1: What BRP Does
The key feature for me was the BRP (Bicycle Recycling Program). This is where I found the motherload of bike parts and friendly advice on all kinds of bike construction issues.