I use Creative Grid rulers, made here in the UK as they incorporate non slip spots on the back of the ruler but for safety I have advised my pupils to have one of these
http://www.creativegrids.com/acatalog/info_1_N0442.html BUT not at that price. In our local "Home Bargain" store they sell the self same item, although they call it a Shower Grip Handle and only charge £2.99/$4.50, quite a difference