Ebay is a possible source,With the caveat of you being willing to open the used box up. Shipping vibration can and will dislodge system ram or expansion cards, depending on age and form factor. Plus you usually have to open them up and check to make sure they aren't dust bunny colonies.

As for parts, Newegg and Amazon are both the easiest, and there are lots of other sources you can find just by looking with a search engine. A couple of the bigger supply sites have closed due to creator death lately, but others are doing fine.

If you decide to test the rendergarden idea, a laptop can be a viable render node. You have to set them up so that they can get rid of the heat generated, but a laptop can be an excellent first node to experiment with.