One place is the clan store if you are in a clan.

If you were fortunate enough when the Snowikings were here,  the charms were sometimes to be found in the treasure chests.

I have heard tell they  are available in packs, but I have not seen such a pack myself, but I think I saw  a screen shot of one 

No doubt others will tell you about other possible  places 

After you have summoned the Shaman,at palace 12, its pretty much like having a second hero, except instead of attacking invaders, the shaman banishes ghosts.

You will need shaman energy to do this.

You also get points to level up from the tasks, and battles.

If you haven't enough charms to summon the shaman, or proceed to the next level,you do not accumulate points.

How do you get the shaman leveled up if there is nothing they can do?

There isn't such a thing as nothing to do. 

There's generally always ghosts to attack.

The shaman gains experience in much the same way as the hero does. 

The shaman also gets experience from the tasks as above. 

There is a nothing to be gained phase, which is when you finished a level and cant get to the next stage. 

At this point you will have to get enough charms for the next one.