So you have 15 dropdown's and all DataSource are same right...
Fetch Data One and Store it in a DataTable and Then Bind all Dropdown's with same DataTable.
In this way you can reduce fetching time of your application.
you can loop each control and Check their type and Bind DataSource if required.
foreach (Control c in Page.Controls)
foreach (Control childc in c.Controls)
if (childc is ComboBox)
if you dont have any caching mechanization. So why we will go for java script we will do in our code behind.