In the Creatives step in the Campaign Flow, you already have the option to go through your Page's timeline to find specific posts. But if you've been using your Page for a while, if you're publishing a lot of posts, or if you need to find a multitude of posts, here's what you can do. 

In the top left of the page, you can fin the usual button with "From URL (or ID)" under the "Find Creatives" dropdown: 

If you choose this option, you'll open a new window letting you input as many Facebook post URLs as you want to:

Remember to put ONE URL per line, and once you've searched for it, if they belong to your page, they will be automatically selected and ready to be used for your campaign! 

You should see the following message appear: 

This is also available for Twitter - you can look for specific Tweets: 

Just click on the "Search by URL" button, and enter the URL(s) of the Tweet you want to use: 

Once you've done so, they will be added to your campaign as Creatives!

There is another search option available that you will have if you are using the labels. Labels usage is described in the article here.