I was looking for this type of controls for a long time, they are very hard to come by. I found that the best source was eBay, and that is your best bet. You have to look frequently and they are about $100 - $150 and up, they do appear from time to time so you can get lucky - be prepared.Hooo-eeee! I had no idea these things were going to be that expensive. My Edson throttle controls weren't that much, and I thought everything Edson made was priced like diamonds. Definitely going to look into the used market... These ought to be well within reasonable eBay shipping costs.
Search for "engine throttle control", filter on "Parts & Accessories" then filter "Boat Parts" as such:
engine throttle control in Boat Parts | eBay