Recently, the Google+ team rolled out an entirely new user interface that was met with both positive and negative reactions. Many features of the old user interface did not make there way (yet) into the new UI. One of those beloved features is the ability to search within a community. As you can see below, the old UI has a search box while the new UI does not.
Here is a quick way to perform a search within the new UI without first reverting back to the old UI. It should be noted, however that the search results will revert you back to the old UI and you will need to click on the Let's Go button on the banner at the top to bring you back to the new UI should you desire.
The quick and easy way to search first is to first find the community you want to search in and open that community. Then you just need to append the following to the end of the URL to perform the search: /s/[Search Term]. You do not have to change the spaces to %20 as the URL will automatically correct the spaces to %20. See the video below as an example.