Firstly, try sticking your "washed" remote in a bag of dry rice for a few weeks (without batteries), leave bag open at top. It may come back to life! Not much lost in trying.
It may be possible to get a 3rd party IR remote from sellers on Amazon/eBay etc., but will be hit and miss, read listings carefully and ask specific questions. Or a higher end "learning" remote from companies like oneforall may work, if you can borrow a working one (take the learning remote into a retailer that has the TV and ask to use the remote in the store to set up yours learning remote).
Not a Roku employee