We book through Costco and they send us through http://www.shoreexcursionsgroup.com/v/a/costco/ to book independent excursions. For a large group, I had great success looking through Trip Advisor and reading reviews to arrange my own excursion via an independent provider, which cut out all the middlemen and ended up being 1/3 of the price of the same type of private excursion through DCL.
It depends on where you are going. It is very easy to book non-Disney excursions in different ports. Most of the time, you can book the same company that Disney uses. I use Trip Adviser to find tour companies in the different ports we are visiting to book independent tours.
Where are you going? I rely on TripAdvisor. 5 Disney cruises and we've never booked one of their excursions.
Too expensive, and we've gotten much better value going with a trusted tour operator we've found on our own.
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