Amazon lists several replacements claiming to be compatible with Roku3 (https://www.amazon.com/s?k=roku+3+power+cord), starting at about $7. I have no experience with any of them.
Roku 3 power supply requirements: 12 vdc power supply supplying at least 1 amp, connecting via a barrel connector, 5.5mm OD and 2.1 mm ID. (Information source: https://community.roku.com/t5/Remotes/Roku-Power-Supply-Guide/td-p/649284)
This describes a fairly common 12 vdc / 1 amp adapter that is used in a number of different devices, so adapters not specifically mentioning Roku 3 will work as long as they meet the same criteria. Pay especial attention to the barrel connector specs.
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Ultra 4800 | Streaming Stick 4K+ 3821 | TCL Roku TV 43S245/C107X