I don't think it is a problem with Panama - it is a problem flying anywhere. If you got lucky before, well, you just got lucky.
These guys have a decent fleet of WW boats in Boquete:
Whitewater Rafting and Kayaking in Panama - Adventure Panama Vacation Packages
They will probably rent you something if you can't get your boat down there, but I don't know the new owner, so can't give you anymore info.
Good luck! I've paddled down there a few times and loved the water and the people.