In your picture, the joystick on the RIGHT is an ORIGINAL Wico and the one on the LEFT appears to be a cheap Chinese knockoff. The telltale sign is that the leafswitches are screwed on instead of popping into the plastic keeper like the one on the right.
I would completely replace the joystick on the picture left (the one with screws through the leaf switches) with another one.
You can find a replacement 8-way (used) on Ebay.
In answer to your question, sometimes you can get replacement Grommets on Ebay. Do an Ebay search for Wico and then wade through the results to see if you can find a replacement grommet auction.