They are called bellcrank caps, and for what it’s worth, someone has 4 available to sell currently on EBay.
Not cheap, but at least available.
Another source might be to scour local thrift stores or the Goodwill for a cast off 60s-70s home organ (Lowrey, Baldwin, etc). These often had similar keybeds that you could plunder for parts, and can sometimes be had very cheaply.