Marble

I'm currently level 174 and have been steadily progressing since starting the game, very well. However, right now I've just hit a stopping point. I currently have 25k gold coins and its only increasing. I can't use my gold to upgrade because I'm out of Marble. I don't see how to get more either, except rarely from legendary packs. Does anyone have any direction for this? I'd like to be able to upgrade my buildings.