I live on a farm and much of the ground is mud. I can't plant grass because it doesn't grow well in the rocky, clay like dirt. I figured that I might be able to plant weeds. They grow deep, are hearty and will soak up a lot of the water that is causing the mud. Is there a place where I can purchase weed seeds?
You don't need "weed" seeds. You need a ground cover or a plant that spreads quickly to keep soil from eroding. Anti-erosion plants include alfalfa, clover, sweet pea, crownvetch, grasses, or vincas--you can buy these seeds in bulk and they also enrich the soil. Go to a farm-supply store or look online.
In the U.S., you should consult with your County Agricultural Extension Agent, who knows your climate and soil, and can advise you best on pasture management.
There's going to be a better solution than just letting weeds grow.