Not sure if this is precise enough. Go into activity and then all activity and in the drop down menu "Actions" chose "sent message", you then get sent messages in reverse order (i.e. newest/biggest number) at the top. You can then scroll through to find the right number. This might be painful if you have several hundred/thousand and you are trying to find one somewhere in the middle. However you could jump multiple pages till you get close and then go page by page.