If you synced your phone while these notes were still on it and you haven't synced again since they were deleted, you may be able to recover them by restoring your phone to your last backup (the backup includes notes). This will revert your settings and data to those contained in the backup (not just notes) so be sure to save any more recent data prior to restoring). To do this:
Without connecting your phone, open iTunes on your computer and go to Preferences, on the Devices tab check "Prevent...from syncing automatically".
Connect your phone to your computer, right-click on the name of your phone when it appears in iTunes on the left sidebar and select Restore from Backup, choose your most recent backup to restore from.
When this finishes you can go back to iTunes>Prerferences>Devices and re-enable automatic syncing.