That's an excellent idea Caryl, never thought about that. I've got a bucket outside full of mosquito larvae and that's also totally green, which I assume is greenwater, it's really old but I think I'll harvest some into a couple of jars and leave them in the sun. I've read you need the water in an area where it will get most sun.
Pegasus has got some info on infusoria/green water here: http://homepages.paradise.net.nz/grumpygr/ (under all about live foods, infusoria)
This site is also useful: http://www.tropical-fish.net/fish_food.htm#Infusoria about getting it started. They don't seem to differentiate between infusoria and greenwater though, from what I can gather the infusoria is microscopic organisms and the green water is just algae water, but you can't have one without the other???
Hope this helps
Now where can I find a disused pool