Valid Licenses for Images / Icons

I had an idea for a theme that swaps out menu text for icons (using the "content" style property). As I was trying to find some icons for throne, kingdom, news, etc, it occurred to me that their licensing might be an issue.

There are a lot of icon websites that offer icons for free, but a lot come with restrictions that they cannot be for commercial use.

I'm not sure if using icons designated for "non-commercial use" would be legal or not.

Any thoughts as to what would be acceptable?

You'd probably have to ask the license holders