Part of the problem is the American Heart Association doesn't recommend going above 104 degrees.
Also, UL doesn't allow temps above 104. I know the manufacturers used to get around this by having a way to go up in temp, but didn't show the actual temp on the control panel. For instance, Hot Spring's control panel used to show UT 1, UT 2, etc. That basically meant 1 degree above 104 or 2 degrees above 104 up to a maximum of about 107 degrees.
They can't do that any longer. That is why you are having issues finding temps above 104. Spas at resorts shouldn't really let you get much above 104 either. They're just opening themselves up for a law suit.
I think you'll find 104 very hot when you are in for awhile.