My sink has a dishwasher air vent that consists of a metal pipe coming up through the hole and there is a hard plastic piece that screws over the pipe. This piece is kind of mushroom shaped with a small dome and the bottom ring of it screws on flush to the sink top. It is located next to the sprayer. It is then covered with a conventional chrome cover. The hard plastic piece has cracked where it screws on and will not stay tightened.
I have been unable to locate a replacement. Any ideas?

What you are looking for prevents air in the dishwasher's discharge line from creating a blockage.

Please go to any hardware store, Lowe's, Home Depot, local store and let them know you are looking for a dishwasher drain above sink.

Go into any plumbing shop and ask for a counter top air gap. If you can't locate one let me give you another option, you can pull the old air gap off and replace it with a chrome cock hole cover. Now you can replace the discharge hose and loop it up as high as it will go under the counter top. Secure the loop with a pipe strap or bamd iron and connect back to the disposal in the usual way. You have just installed a high loop air gap.
We do not use counter top air gaps in my area, too much service and backups. We have never had a complaint on a high loop installation. Just a suggestion, Tom