Apple - What do I type to produce the command symbol (⌘) in Mac OS X?
If you're just looking for the Unicode versions of Mac OS X keys, you can use this Apple support document to copy and paste them:
Mac keyboard shortcuts
https://support.apple.com/en-us/HT201236
⌘Command (or Cmd)⇧Shift⌥Option (or Alt)⌃Control (or Ctrl)
More generally, Mac OS X provides a pane to insert special characters. You'll find it under Edit -> Emoji and Symbols in any program that takes text input. The Command key symbol can be found by searching for it's name "place of interest". To insert the character, double click it.
If you're really hardcore and are looking for a way to type the character by entering the Unicode hex code, this is possible:
Go into System Preferences -> Keyboard -> Input Sources, click "+", scroll to "others", select "Unicode Hex Input" and click "Add"
From the input source selector in the menu bar, select "Unicode Hex Input"
To enter a Unicode character, hold down option and type the 4-digit hex code for the character and it will be inserted. In this case, it would be option+2318.
An answer for OSX Lion (circa 2014):
You can also type ⌘+^+Space (aka Cmd-Ctrl-Space) to bring up the characters menu, then start typing to search, and search for place of interest, then press enter to start selecting them, then tab to the correct result, then enter again to insert it.
Sounds like a lot, but it can really be narrowed down to ⌘+^+Space, typing pla, and then two presses of Return.
I also found it hard to find a straight answer for this, so I just went into System Preferences > Keyboard > Text and created a use symbol and text substitution option where when I type (cmd) it replaces it with the command symbol.