It can be found in the only accessible room on the first floor (filled with bark scorpions) in the El Rey Motel. It is possible that the bark scorpions in the room are the "consequence" described in the note.
Your debt still remains unpaid. We have warned you twice. There will not be a third warning, only consequences. You have one day.
The note cannot be picked up and put in the inventory, only a text box appears when interacting with it.