WHSmith Whiteboard Markers. Pack of 4 (Red, Blue, Green, Black) for £4.99 from a well-known High St. retailer!
Almost as good as the CFS jobs, but don't make you quite so high in a briefing cubicle....
Best thing for cleaning the board afterwards was one of those magic cloths which in-flight used to supply. Bung them in the washing machine now and again and yet they last for ever!