How can I cultivate and store yeast?
In order to store yeast for that period of time it has to be make inactive, and the process to do that yourself is challenging, time consuming, and you'd need specialist equipment. It would be prohibitive in both time and cost, far above just buying yeast. 

You can of course make a starter by mixing equal amounts of flour and water plus some yeast, this will keep the yeast alive by feeding it, and you can keep it alive indefinitely by feeding it once per week. You add some of the starter to your bread and it supplies active yeast.