I was going to suggest what "1 of 5" already has, dip in tool handle grip rubber from the hardware store. If you dip, it will require several coatings to achieve the proper thickness.
You didn't indicate the correctness you desire, "show quality" or "make do". I assume the former but include several of the later category for a temp solution.
1) Mask off metal lever and paint 1-1/4" of end black.
2) If you want some thickness you might try several layers of shrink tubing, applied one at a time, making the last layer just a bit longer so it hides all the other layers. You won't be able to cover the rounded end of the lever but if painted black should look OK.
3) Although the item you're asking about is rubber not plastic it could be made from 3 layers of acrylic. Assemble pieces, round off and then flame the edges. If done correctly could look very close to original.
I purchased a rubber repro set just a few years back so they may be available somewhere. I don't recall the source of my pair but possibly eBay.
Growler: Purr...fect