To list the current bounty questions, click on the featured tab.
On the current UI (2019) it's at https://stackoverflow.com/questions
(for example) where you'll see this:
There doesn't appear to be a way to limit the search though. There's no hasbounty:
keyword on the search page. Since September 2011, the hasnotice:1
search option can be used to show bounty posts, but that also shows other posts with notices (deleted question).