Key combination for Apple command key?

You can find it in the Special Characters palette, Technical Symbols.

You an also see if your keyboard layout supports it. If you show the keyboard viewer in the menu (from Keyboard System Preferences), it will show your keyboard layout as you hold down the modifier keys. I don't think there is a shortcut for it, though.

If you use the Unicode hex keyboard, opt+2318

I think it's called the "Place of interest sign" in unicode. I believe its possible by using direct unicode entry (its hex code is 2138), but it's more trouble than it's worth and I can't remember how you do it.

Anyway, OS X has a pane for special characters, in most applications its normally

Edit > Special Characters

The ⌘ symbol (see what I did there?) can be found under 'Technical Symbols'. You may need to use the gear wheel to customise the list by adding

Symbols > Technical Symbols

Once you have one instance in your document, which you can copy and paste from the example above, you can just copy and paste it.

C.