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
BikeAthens (Athens, GA) has a modest membership fee of $30 a year (http://www.bikeathens.com). They are interested in all kinds of alternative transportation issues and work with state and local agencies promoting their agenda.
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.