Just regular washers that fit on the screw rod.
https://www.amazon.com/gp/aw/d/B016PNN2TG/ref=mp_s_a_1_5?qid=1461919776&sr=8-5&pi=AC_SX236_SY340_QL65&keywords=plastic+washers&dpPl=1&dpID=41FuFo8EQTL&ref=plSrch