Hello,
In order to find the button, you can use "getElementsByTagName" but prior to that you should select the class/id before the button and then build your way down until you reach the button by tag.
Another solution is to use selenium library instead of IE as it works with modern browsers (Chrome, Firefox) and have lots of extra functions, like Xpath, which is exactly what you need.
Please open a chat with me so we can deal with this issue together.
Kind regards,
Sorin