i have the exact same thing the ER20's as i do DJ photography at festivals, i use the macks brand of those particular earplugs called hearplugs with my internet research they seem pretty popular: http://www.macksearplugs.com/products/hear-plugs-earplugs , i wash them with soap and water, or anti bacterial gel, cloth etc...
http://www.macksearplugs.com/products/hear-plugs-earplugs
http://www.macksearplugs.com/products/hear-plugs-earplugs