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.
Step 2: How I Started Volunteering
Most BRPs (also known as a Bike Co-op) use volunteers to recycle and restore bikes. I started by cleaning and tuning up kids bikes for their Christmas program. I fixed up a dozen bikes my first year with BikeAthens and these were distributed (along with about 100 other bikes) to kids who otherwise wouldn't have had a bike for Christmas. They recycle kids bikes all year for distribution to needy families.
I have also helped clean and repair bikes that BikeAthens sells or gives away to needy individuals. For example, they help individuals in local DUI programs and homeless individuals.
Step 3: A Working Relationship
I eventually became a parts harvester. I have some metal working skills and tools. That filled a need at the BRP. I would take unrestorable donated bikes and strip them for parts. Then I would cut up the frames for various projects at BRP. Finally I would help by hauling the leftover metal parts to the recycling plant and sell them for scrap. The BRP got the money.
In return for my help they let me select used parts I needed for my homebuilt projects.
Step 4: Find a Similar Program Near You
There probably is a similar program near you. Ask your bike-riding friends if they know of a BRP or local Bike Co-op nearby. Chances are they do. If your friends don't know of a BRP then do a Web search on the terms "bike recycling" and "bike rescue" with the name of your town and/or state included.
If there isn't a BRP near you then maybe you should start one. It is a great way to go "Green" and get parts too.