You can get Plexus at a plastics or plexiglass dealer. It is made to clean plexi, clear vinyl, etc. VERY good stuff.
Here, boaters world carries it too. *;)
http://www.boatersworld.com/product/176970838.htm
My dad use to do allot of plexi work like building church pulpits and such. He swears by that stuff. *8)