You can get DT's Phytoplankton, it is concentrated but also a bit costly. Or:
I buy mine off of ebay (search for phytoplankton).
This weekend I'm gonna pick up 2 10 gallon tanks, one for raising artemia and one for propogating Zooplankton. Then I won't have to buy zooplankton anymore.
Anyway the Artemia (brine shrimp) are teeny filter feeders and like to eat zooplankton so wih the tanks together I will just scoop over a measured amount to the Artemia when they need it.
Anyway I digress.
When I started adding Phytoplankton to my SW
tanks I noticed a positive difference in the anemones and corals and stuff, as expected. 8)