OK, I saw the posting. I have the screw in type.
I purchased a brass one for $20 from a back yard boat dealer who has been in the business since the 60s.Another choice is to go to a plumbing supply store and buy a plastic threaded one. I don't have the required thread size at the top of my head but will try to get that to you. You will just have to drill the small holes to drain the water when the plug is up.
There have been a couple listed on eBay recently, however I have to caution you as one sold for over $70. Not sure why someone would pay that much, but good for the seller.