I don't think you can just get the stick, you'll have to get a whole new set in a box. If you had an older one, you should be very happy with the performance improvements in the current Streaming Stick+ (3810).
Roku Community Streaming Expert I am not a Roku employee, only a user like you. Please, no support questions via private message -- post them publicly to the Community where others may benefit as well. If this post solves your problem please help others find this answer by clicking "Accept as Solution.".
Streaming Stick 4K+ 3821 | Streaming Stick+ 3810 | TCL Roku TV 43S245/C107X
Is the above advice still the only option? I had a streaming stick+ (HD / 4k) but lost only the stick (we probably left it in a hotel room) but I have all of the other parts. It seems a real shame and a waste of perfectly good devices to toss the remote, power cable and adapter in the garbage (tho' I could try to find a local e-waste recycler). I don't mind paying for it but in the interest of environmental impact, I would think the company would be willing to replace just the stick.
Given how often you see posts about lost remotes on this forum, I think it would be easy to get rid of a spare remote. Likewise, when I searched for roku on ebay, I saw several Rokus for sale with missing remotes.
By the way, if you haven’t already, I would go to your account and unlink the lost device before the next owner/finder starts messing with things.