Depends on how unique the thought needs to be. You could simply count up from 0 and eventually you'd encounter a number you'd never thought of before, like 145,398, which I'm pretty sure is a number I've never thought about. You could do this forever and still be having technically unique thoughts.
The problem, I think, is that all thoughts seem to be a product of previous thoughts. Totally new thoughts are driven by external stimuli. We can, however, mutate an existing thought into a new one. I think this would be my process, take a common thought and change one aspect of it at a time until it's unrecognizable.
That's my interpretation of the creative process behind fiction and invention also. You take a familiar concept in your mind and mutate it with the goal of either making something more efficient or effective (invention) or of creating engaging narrative (fiction).