I lost the cap for my sprayer last year and needed a replacement drain cap. After searching high and low I couldn't find anything in store that fit.

The threads are 19mm which comes out to around ¾" which seems to match some name brand sprayers.

Though it threads on perfect and seems to fit I believe the original cap had a grommet. To get around this, I used some Gorilla Glue silicone sealant though any should work (make sure it's a removable or soft sealant so that you can remove the cap when you're finished).