How did Caster find Saber?
There is probably no canon answer, but we can speculate.


  and both him and his Master didn't operate like proper participants in the Holy Grail War


This is actually one of the reasons why he could track Saber. Normal servant would focus on all other masters equally. Caster on the other hand single-mindedly focused on Saber. He could invest all of his time and energy looking for her and following her. This is probably why Rider was able to enter his workshop without being noticed.

As for Einzbern's bounded field. I think it was designed against humans and low-level mages. For something like Caster, it would be child's play to break it or to ignore it. And even if it did pose problem for him, he just needed to see Saber entering the area. He doesn't need to know where exactly she is in the area. That's probably why he lured her out and not approached her directly.