What is the name and model number of your roku? If it can be paired to a wifi roku remote those can be bought locally from walmart or best buy. If it only takes IR remotes then cheap universal remote would do from taget or the dollar store as long as it lists roku. If its Roku tv, walmart has the One-For-All urc-7935 streamer remote that works great with roku tv's.
But I would go with cheap universal remote, benefit being you would be able to control the tv volume and power along with roku, unlike the your original roku remote and the one above. It could cost under $10.
They all come apart. It just takes the right tools or really good fingernails. Your easiest option is to get a cheap universal remote that supports Roku. You can also use the Roku app on your phone as a remote.