bbPress has simple search built in using keyword search and simple form with the single input field. But, this can be also disabled.
If the bbPress is not showing the search form for forums, search widget or the dedicated search page is not found, that means that search is disabled in bbPress. To make sure that search is enabled, open WordPress admin side Settings -> Forums, and find the Search in Forum Features section. Make sure that ‘Allow forum wide search’ is enabled. Once you do that, just open WordPress Settings -> Permalinks page to make sure that all rewrite rules are regenerated and new search page active.