Abigail,
Go to the link that I have posted in my source. Once there, select the 'MORE INFO' button that's located at the top of the page, just right of the text "Selected Parts Listing". A window will pop up with text saying "Illust. 1 of 2". Hit the 'GO' button directly to the right of it and you will see a detailed diagram of the Sienna's radiator system. The Thermostat would be Ref #16331A in this diagram.
Hope this helps!
www.toyotapartseast.com