Some of my personal favorite sources for rope:

short descr diameter strength $ / ft source notes
paracord 3mm 400lb breaking $0.09 / lb REI I really like practicing knots in this. It is very dense and doesn't compress much, so it's very easy to undo knots that would otherwise jam. It has the unusual ability to be PUSHED back through a knot, because it's so dense. (rope can almost never sustain compressive forces)
braided fishing line (Dyneema) 0.4 mm 80lb breaking ~$0.04 / lb any fishing store This stuff is high-tech and amazingly strong. Major downside: it's high-pressure, so it will cut through many things: wood, human muscle, other braided fishing line, it doesn't care. Put something in its way, and it wants to cut it. This often take some extra design work to accommodate.

Note: It's important to distinguish between breaking strength and the working load limit. Some packages quote only one or the other. I quote whatever is given on the package.