Yamaha Thailand sells the Yamaha YZ450F, not all Yamaha dealers know that. But they have parts for the YZ450F 2015, 2016 and 2017 models.
You did not specify which year your motorcycle is, therefore I included the YZ450F Water Pump parts catalog for the 2015 model. The part catalog in in English and Thai, which makes it easier for you to communicate with a Yamaha dealer.
You can also try to call Yamaha Thailand, not sure if you in Bangkok or somewhere else, but the main telephone number is 02-740-8000. They can help you, and speak English… something what is still rare at a motorcycle dealer in Thailand