Thanks for your first post in the Roku Community!
We understand that you are inquiring about the remote that is compatible with your Roku 3900RW.
Please be advised that your Roku Express 3900RW is compatible with a Roku voice remote such as a Roku Voice Remote Pro, Roku Voice Remote, or Roku Enhanced Voice Remote. All are available for purchase at the Roku accessories store.
For future reference, here's how you can How to set up your Roku voice remote | Official Roku Support
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