Maybe for the first time he steals(or something similar) he can get the ability to steal abilitys (like magic) or instead of an ability he can get a 1 time ability that way he can do magic. Just a suggestion.
You could travel to other worlds and use criminals.
Still, 100,000 souls, that's a lot. I have no clue how I can go about getting that many souls, I may be a Daemon, but even I'm not comfortable going around massacring hundreds of thousands of people.
I haven't found any complete adventure time fanfictions, but I do like one on this website called the Fifth Element but this one is probably better, I hope the author continues this novel.