Maybe the best way to get error notes is to order bricks of notes (if you can afford temporary of). You usually have to pay for, and you have to be patient to sift through. The odds are slim but exists to find errors, most of which you find are minors, but you can get a bigger. I know somebody who had find many nice errors by this method (in many years).
The law of great numbers makes that you're sure to find something good with the time (and the numbers). The more you look to notes, the higher are the odds to find some nice errors.
An other way is to tell to your friends that you're looking after unusual looking notes. Maybe someone will bring you someday an interesting note he got from an ATM...
Remember, it's a hobby: you should have some pleasure to do this kind of search.
And it's easy to get discouraged after few bricks without finding anything more interesting than the radar note
. So keep going...