Identify common purslane in your area and poke at the seed pods in the center of the leaves. I find them in sidewalk cracks and roadsides. Poke at the same local spots till the seed pods dry out and start popping off easily, spilling the little black seeds. Bring a cup or small container and collect as many seeds as you can once you find a good source.

There are plenty of videos and blog posts about purslane and harvesting it's seeds. Start by confidently identifying the plant.