Where do I find or get Blue skill box to further strengthen troops in strengthen phase?

You might mean the 2nd tier supply box required when you try to "Promote" your officers right? If so, these can be obtained by fusing lower tiered supply boxes of the same type. Usually, when you click the icon, it will open up a new window that will show you how many lower tiered supply boxes you can fuse and how much of the new item you can make. Note that this becomes really very hard when it gets to the 4th and 5th star promotion.