I buy my 'Instant SAF yeast' in a white package with blue lettering at Sam's Club. I bought a package of it (it's fairly large) and put it in a quart mason jar with a screw on plastic lid. I store it in the freezer. I just remove whatever I need for baking, and keep it in the freezer.
I bought my last package in 1999 or 2000, and it's still very active when I use if for making bread and have had great results! The freezer keeps it fresher. Just use warm water when you reactivate it or let it come to room temp (I just toss it in with my flour/water mix and let it activate at it's leisure as I'm usually making a 'sourdough sponge' and the stuff it sitting around for at least 2-8 hours).
Here is a link to the SAF website if you want more info on the yeast: