We booked a Juneau whale watching/Mendenhall Glacier combo cruise independently. I think it ended up being with the same company used by the Disney Port Adventure. We decided to book independently so that we had more control over the order of the events, to allow us to have more time at the glacier than the Disney excursion we were looking at would have allowed. The added bonus was that it saved quite a bit of money. We used Juneau Tours & Whale Watch (
https://www.juneauwhalewatch.com/) and booked directly with them. No complaints. I'd book with them again if returning to Juneau. As to how I found them...internet research...Google, TripAdvisor, looking at some of the companies third party websites for Alaska excursions.