I don't think anybody sells them now. It's a pretty simple procedure to put a groove in an existing one with a cutoff wheel on a Dremel tool. It doesn't need to be deep, or precise. Ditto for relieving the pushrod cup in the lifter to allow air to escape from the body- - - - -a shallow cut to allow a tiny seep leak is all that's needed. Smith Brothers can build just about anything you want from scratch.
www.pushrods.net/Jerry