There already is a search option. On Steemit, go to Explore Communities and at the top you will see a search field. On Steempeak, go to Communities > All Communities from the menu. Then scroll down to the Popular Communities section and you will see the search option.
Is that what you were looking for? I guess the search options are not very easy to find.
Those search function are not only hard to find they are very very slow and unintuitive. If there is no result I'm never sure if the search has finished, is hung or what I'm looking for doesn't exist.