How did gandalf get his staff back?

[Inziladun]

Welcome, Dead Mike!

I would agree with Foley that Gandalf's staff was probably not taken in the book.

For one thing, I don't think Saruman needed to do that in order to overpower Gandalf; as the head of the Order I believe Gandalf was subject to his power.

Also, in Frodo's apparently retrocognitive dream in the house of Bombadil, Frodo saw:

...a vast tower but not made by hands. On its top stood the figure of a man....the figure lifted his arms and a light flashed from the staff that he wielded.

FOTR In the House of Tom Bombadil

That isn't definitive, but I think it doubtful the detail about the staff would have been written as seen by Frodo if it hadn't been true.