"I'll have to have a think about it and get back to you."

or, similar

"That's needs some thought. I'll get back to you about it."

.

I think there's no avoiding "I'll get back to you" or something substantially similar. After all, the whole point is to tell them that that's what you're going to do.

But "I don't know" is the bit I think you have a problem with. It seems to suggest that someone else would know: if only they were here, they would be able to just "look up" the answer from the right corner of their brain. But you want to communicate that this is a problem that actually needs solving, whoever is doing it. "needs thought" is the best I can think of for that.