Squire James
Explorer
It's pretty obvious that Gandalf could have simply talked to Elrond about the basic historical details about what Isildur did with the Ring. He was standing right there after all! The book account was not much different from the movie here, though perhaps Elrond's advice was delivered on the way back to Gondor instead of the Crack of Doom itself.
What Gandalf wanted, though, was very specific information about what the Ring looked like, and how it could be distinguished from any other plain gold ring. For that he needed first-hand information, and that meant Isildur. He ruled Gondor for a little while before his final trip north, so Minas Tirith was a pretty good place to look. I'd think Osgilith would have been better if it hadn't already been trashed by the Enemy, but Gandalf worked with what he had.
What Gandalf wanted, though, was very specific information about what the Ring looked like, and how it could be distinguished from any other plain gold ring. For that he needed first-hand information, and that meant Isildur. He ruled Gondor for a little while before his final trip north, so Minas Tirith was a pretty good place to look. I'd think Osgilith would have been better if it hadn't already been trashed by the Enemy, but Gandalf worked with what he had.