Yeast is a living organism. It can be refrigerated and dried, but everything I've read seems to say it won't last more than 2-3 years. I don't know about the mylar bag storage.
Got this from my sister.
Yeast Starter similar to the sour dough starter I got from one the people here (sorry, with my memory I don't remember who)
Ingredients: makes about 4 cups, ready in about 3 - 4 days
2 cups all-purpose flour (organic seems to work a little better)
2 cups bottled (non-chlorinated) water (very important, will kill the natural yeasts)
1.Mix flour and water and stir well. Cover with a light cloth and let sit on the counter until it shows signs of bubbles (the yeast is growing).
2.To use just take out the necessary amount for your recipe. Yeast is a living organism and needs to be fed. When you take some for your baking needs, you need to replace that amount with equal parts flour and spring water (if you use 1/4 Cup, replace 1/8 Cup water and flour).
If you keep your starter at room temperature it should be fed every other day, and if refrigerated, fed weekly. First mix up about 1 level tablespoon each of flour and water and gently mix into the original batch of yeast starter. As you feed the starter, it may make more than you want, just dump some before you add the feed mix.
Try this recipe before you need it and try the amounts of yeast in the recipes, it may need to be adjusted. Once the starter is growing well, refrigeration makes it easier to keep. But as it shows, you don't need to store large quantities of yeast.
I keep it in the freezer, lasts for years. Worst case scenario you may have to bake bread daily and save some of the dough to mix in, the yeast will reproduce if you keep it warm and have enough sugar in it for it.
yeast is also present on the skin of fruits and vegetables. That is how brewing was originally discovered. Floating say apple peels in water for a few days will result in a yeast solution, they call it yeast water. There is also the landic method of brewing, in which cobwebs are harvested for the natural yeasts that collect in them. I never have tasted a landic beer and I don't intend to.