[ga0bi]

For those unable to find it in the iOS App Store: Search for a Google app (i.e. Google+) and click "Related". You'll find Hangouts in the See All list.

[orbitur]

Ran into this and solved by searching for just "hangouts".

[tagabek]

I could not seem to find it through both methods. I ended up going to the Top Charts and found it as #9 in the Free section.