A good rule of thumb is to try and send out the your save-the-dates about 5 months before the actual wedding day.
Many of the planning couples I speak to use the 5-2 method for remembering when to mail out their wedding notifications:
5 months - mail out save-the-dates
2 months - mail out wedding invitations
An exception to this rule is if you're having a destination wedding, try to mail out your STDs at least 8 months before the actual event date. This gives traveling guests enough time to plan flights, hotels and make out of office arrangements.
Recommended websites for finding unique save-the-date announcements are: