I put my holders in pink anti-static bags (reduces dust-attraction from static charge), then place holders in the bag with long side down so there is no tendency to pull the slide inadvertently when removing them from the bag. Another popular option is to use rubber bands (wider than most) to put around the holder so as to hold the slide in. Try stationery stores, they usually have an assortment of them. Replace them whenever necessary as they don't last forever.