IMPORTANT: If you've had a Smart Noise account before March 1st, please disconnect and reconnect your Facebook integration in your Smart Noise account settings for this to work.
On Smart Noise you can run ads with existing Instagram posts. You'll see a 'Use existing Instagram posts' option at the 'Creatives' step of every ad campaign:
All that's required are two steps:
1) Have an Instagram Professional account. Follow this guide to switch your Instagram account to a Professional account.
2) Connect your Facebook page to the Instagram account. Follow this guide if you haven't done it yet.