Usually Justwatch should be able to tell you where something is available. However I will note that it's remarkably common for something slightly old to just not be available anywhere. You cannot watch Das Boot in my region on any service so far as I can see. So I guess you loan from the library or buy DVDs off of ebay or something?

Justwatch for me is just showing "Stream this or pirate it?"

I love JustWatch and most of the time it's given me the answer I need.