How do I get a note that I deleted off my i-mac back?

You can restore from a Time Machine backup > Time Machine - Frequently Asked Questions 15. How do I view or restore selected items?

Or, if you have iCloud enabled, try deselecting Notes in System Preferences > iCloud then reselect the box.

Give iCloud a few minutes to re sync the notes.

Aside from that, you may be out of luck.

Triple-click the line below on this page to select it:

~/Library/Containers/com.apple.Notes

Services ▹ Reveal

from the contextual menu.* A folder should open with an item selected. Quit the application if it's running. Restore the selected item from a backup that predates the unwanted changes.

*If you don't see the contextual menu item, copy the selected text to the Clipboard (command-C). In the Finder, select

Go ▹ Go to Folder...

from the menu bar, paste into the box that opens (command-V). You won't see what you pasted because a line break is included. Press return.