Yeast packets

Hello,

Sometimes I use Rapid Rise yeast that come in the 3 packs, and when Im working with recipes that dont use the entire pack, I try to save the left over yeast by "tightly sealing" the packet back up as best I can.  But when I go to make new bread I'm hesitant to use my left over yeast because it had been opened, and end up tossing it anyways, and opening a fresh one.  Does anyone have any tips or helpful information on storing the leftover yeast that comes in the packs?  I do not see in the grocery stores that I can purchase Rapid Rise yeast in any other forms.