1. A pair of cleaner shrimp will produce plankton at least once a month.
2. You can buy cultures of copepods, amphipods and marine rotifers. Build a pile of live rock rubble in a corner of the tank or in your refug and feed the system "green water" regularly.
You have to be careful of the type of introducing "wild" plankton into your system, you never know what could start growing. The above two options are the easiest and safest ones I can think of off the top of my head.
People use DT's phyto to start their cultures, you can also find Plankton cultures on-line. There was some one here a while back who made a plankton systems.