How can triiodide be obtained from iodine?

I am conducting an experiment in school which requires triiodide. I know that I2 + I- <--> I3-, which is iodine + iodide=triiodide. what im not sure is if the school lab provides triiodide and if they dont, are there complicated chemical procedures required to obtain triiodide from iodine? So how can i obtain triiodide then? or is it plain mixing iodine and iodide?