any idea where to get the bronze key to the glass palace front door?

Yes, it is located in the swamp. You can get there before of after you encounter Cagen, I did it first. As soon as you enter the swamp (you enter with your party facing you/the screen), you can go left or right. Go left and you will encounter a boss. Defeat him and you will get the Bronze Key as well as the Lightning Staff. Now you can enter Glass' Palace.

Note: The place you enter is the screen after the fire tree.