I know you can get an abundance of easy soul gems in the cairn. You can even fill them by finding the crags in the ground that emit souls. However in my latest trip through the cairn I sneaked up on the first bonemen that spawn. Since they didn't see me right away they began chasing after one of those floating orbs, running back and forth and attacking.

• Are you able to collect those floating orbs? I thought they were just ambiance.
  
• Is there any other way to fill soul gems inside the carin?

- Just ambiance

- I don't know. Someone else will have to answer that.

I don't know of any other way of filling soul gems apart from killing soul-trapped creatures, or killing them with soul tear, i guess that works on the Xmen in the soul cairn too

And the floating orbs, they are lost souls, or rather, the unused energy of lost souls