These are handy to have on hand when you need to temporarily hold threads or cords together. They also have another useful function. During the preparation of a beaded braid, it keeps your beads from falling off the end of the cord. Spring stoppers are available in different sizes and some have plastic ends that are a bit softer on your hands.

Link to purchase Spring Stoppers ($5.50 for 8) at KumihimoResource.com

Alternate Solutions:
Stop Bead: This is an easy way to secure beads on a cord. To make a stop bead pass the cord through a bead. Slide the bead down the cord to about six in. (15 cm) from the end. Pass the thread through the bead again, in the same direction as the first time. To make it easy to fit the cord through the bead on the second pass pull the cord over your finger with the bead on top of your finger. This will make the bead hole easy to access & more visible.
Note: The stop bead needs to be a shiny bead so that it will slide easily on the cord. When using micro cord use an 11º bead and use an 8º bead for Tex 135 or Tex 210 (bead cord).

Make a loop at the end of the cord: When making beaded braids and working with CLon or SLon another easy solution is to simply tie a loop at the end of the cord. I feel a loop works better than just tying a knot as the loop is larger and keeps even big beads securely on the cord. Also, when working on a marudai the loop can be used to secure tama to the braiding cord instead of using a leader on the tama.