Any antenna will do if you are just trying to pick up a few local over the air (OTA) channels. I haven't seen specifically a "Roku antenna". Try seeing how a $10-20 RCA antenna performs for you from Walmart. I am pretty rural and a very old rabbit ear type works fine on my kitchen TV. (A little fiddling here and there). The rest of the house has coaxial type from my attic antenna.
Roku makes streaming devices requiring an internet connection.
Roku Community Streaming Expert Just another Roku user... I am not a Roku employee.