Like @RoyB mentioned most of the third party remotes don't seem to work on the NOW branded devices from reading reports on the forum by other customers (unless a forum member have any experience of a full functioning third party remote which works with their NOW 4K Smart box or NOW Stick).
The problem going down the second hand route is that you got to find a trustworthy seller who doesn't sell you a broken remote and even if it works there is no guarantee how long it will function / last.
If you do decide to gamble on a second hand remote (don't pay much for it) then make sure it's a branded NOW "smart voice" remote like the picture below.
It can be either all black like the picture or have pink direction and enter buttons on it with the NOW TV logo in pink or the NOW Stick voice remote with the yellow direction and enter buttons with the yellow NOW TV logo on the bottom of the remote should also work with the NOW 4K Smart box with the pink logo on the box top casing.
You could try the replacement £19.99 Roku remote which Argos sell, but before you buy make sure Argos offer a no quibble return full refund policy if it doesn't work with your box (never tried this product myself on any of my NOW branded boxes or stickes).
Perhaps a forum member who has experience of this can confirm either way.
Personally myself I would bite the bullet and purchase a new Roku player.
