I found a few old rolls sitting outside an apartment complex in a college town with the word "FREE" plastered on the sides in standard black sharpie. Had some measurements on them too.
A very old church in a nearby town was being demolished, and so we gathered up all the carpet out of there too.
And, I have also contacted at least three local places that sell and install carpet, and told them I would love to have the old throw-away carpet when they do re-flooring jobs.
Got to know your sources, of course.