I have a very small property and rely on neighbors' excess biomass (weeds), of which there is always plenty. Even if you are in a rural area, I bet you have neighbors with an excess of weeds, and then there are the roadsides and parks... I have found giant ragweed to be particularly excellent for sheet mulching, and literally nobody wants it to bloom...
http://blueboathome.com/blogs/ben/sheet_mulching_ragweed
so my advice would be to look for ragweed patches and find out who owns them. You can fill a pickup truck with ragweed in no time, and if you harvest it just before it would have bloomed -- July, here in Kansas -- it will not come back the following year, so it's a win-win. Ragweed's close relatives
sunchoke and sunflower do not break down as quickly or thoroughly in my experience, so look for the raggedy leaves.
Other invasive weeds that are easy to harvest and make good mulch in my experience include trumpet vine, bindweed/morning glory, and honeysuckle. All of these are easy to recognize from the road once you get a good look at them. Good luck!