Any recommendations where to get German-style quality windows in Thailand?

You may have a look

 

http://deku-upvc-windows-thailand.com/

I suggest look at something like this:

 

Home - Architectexpo

 

If you can go to the show that is best. But if not then look at the information on the website.

I found a supplier for sliding doors that way. I am sure you will also find suppliers or windows.

 

It seems that many importers or distributor don't want to work directly with the end customer. They want to work with dealers or construction companies. Anyhow, it will give you an idea of what's available.