I read an article on an airline forum that you can use your own antigen tests for the proctored tests. Even the ones you get free from the government. There are even companies that ONLY sell the proctor part so you can pay less and use your own test. In a pinch, if you have any other antigen test, I am pretty sure you can use them.
Found the article:
Use your own Covid Tests
Companies that just sell the proctored part are iHealth, MyLabsDirect, OnPoint Testing and Total Testing Solutions. The article was about getting tested before returning to the US which is no longer necessary - but am sure the same would work for Disney. I'm going to try it for my cruise in December. The "proctor" part only is less than $25 but you do have to make an appointment.