I belong to a bike co-op shop, so I can source my used parts from their 'stock'. The shop takes in donated bikes, and either fixes them for resale, 'sells' them to members for them to rebuild, or strips off the good parts and scraps the rest. It can be a hassle to find the right parts, but you can find most if you look hard enough. Last week I fixed 2 broken spokes and re-trued the wheel for a cost of $0. (I also needed new rim tape, so I got out with a 'bill' of $1.)
If you have a co-op nearby, I would highly recommend checking it out. I not only get access to used parts but all the tools I could need, and there are people there who can teach me how to do anything I might not already know.
If you don't have a co-op, you might be able to ask around the local shops what they do with unwanted parts. Things like platform pedals are often switched out for clipless, and they might be willing to sell you their extras cheap.