Beats me.Hmm…I am not the best person to answer that.
I can find out for you.I can’t remember off the top of my head. I’ll get back to you on that.I don’t have that information here right now.
I don’t know anything about…I have no clue/idea.I haven’t/ haven’t got a clue.
I haven’t got the faintest idea.I haven’t had time to think that through yet.I haven’t looked at that yet.
I want to be sure and give you the correct information. Let me check.I’ll double check and let you know.I’ll find out and let you know.  
I’ll get back to you on that one.I’m going to investigate that further.I’m not 100% sure on that.
I’m not sure / unsure.I’m probably not the best person to ask for that information. I’ve been wondering the same thing!  
I’m afraid. I have no ideaIt could be one of many possibilities, I’ll look into it.It’s a mystery to me.
It’s beyond me.Let me check on that.Let me find out for you.
Let me look that up for you.Search me.That requires a bit more research first.
That’s a really good question, I’ll check/ find out.That’s not my area of expertise, I’ll ask …..That’s something I want you to find out for yourself!
That’s exactly what I’m seeking to answer.That’s good question, but I don’t know.That’s good question, I’ll check this.
There are several possible answers, I’ll need more information first.What do you suggest?What do you think?
Who knows?