Mage-Specific House Rules/Buying Spheres

Mage 20 implies that a mage has to have a teacher of the same focus to raise spheres, i.e. someone from your Tradition, Convention or sect. However, this is supposed to happen during downtime on a tabletop campaign, and we are a 24/7 campaign. Thus, it's simply not effective and will lead to headaches.

So, yes, you can learn spheres without a teacher.

However, to reproduce the complexity of learning them you can only learn one sphere per RL month. So choose wisely.

As a side not, if you have the flaw Sphere Inept, the sphere you take the flaw in can never be your lowest sphere in number of ranks. Which means you have to start out with at least one dot in it.