We generally make them.  Buy a skein of dork green yarn.  Boil or otherwise soak in hot water for a while to make sure the dye doesn't run.  I generally don't bother with artificial materials -- the dye is fast.
After the yarn is dry, select a form on which to wind your mop.  You can use a book, a piece of plywood, a clipboard or anything else.  Wind about 50 wraps around the item and secure at one end by tying all the strands together.  Hold the multi-strand loop by the place where it's tied and cut the strands apart directly opposite from the fastening.  Attach a float (35mm film cans make good ones) if you want a floating mop.  If you want a sinking mop, just drop it in.