I find a lot of 2XL easy to find, however larger can be difficult to locate. In the past I've asked what looks like a non-college program CM if they have my size and the next day there is some out if you happen to see somethign you like and know you can circle back the next day (like your doing 2 days at MK for example). Certain things like the country brands found around epcot may never have them as they tend to not have larger countrymen so they just don't make it above 2XL. Keep in mind those are regular brands that you can usually order somewhere online if it's not disney branded. (some of the disney branded is standard though so don't count it out).
Another interesting trick is using the
shop disney parks app. Not everything is in the app but you can scan the bar code, order it from the app in whatever size you need (usually, not everything has larger sizes). When standing in the park, there is options to have it waiting for you at the front of park (need to order/pay early in the day), your resort (takes a couple days to get to the resort as normal), or shipped home
The shipped home works from outside the park as well including after you get home. Also I tend to find the merch on the shop disney parks app to also be on
amazon or other stores for less money, even the exclusive park only stuff.
And last, take a picture and ask guest relations where you can find it when you go to exit the park that night or happen to be passing it. It's not well know but you can actually call them after you leave to get anything from the park mailed to you for a service fee.