Here are the steps:
1. While in the game, press Ctrl+Shift+C all at the same time. This will open the cheats window. Your screen will get a little darker and you'll be able to type in the upper left corner.
2. Type "testingcheatsenabled true" without the quote marks, then press Enter.
3. Repeat #1 above.
4. Type "buydebug" without the quote marks, then press Enter.
Now when you enter Buy Mode and Sort by Function, you'll have an extra category with a question mark on it. This is where you find all the special Buydebug items.
(Edit by Hosfac to fix the error
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