At MK, I'd check The Emporium and shops in Tomorrowland. DD bought some Capt Marvel stuff at the space mtn gift shop, but this was several months ago. When Guardians of the Galaxy movies came out there was merch at Mickey's Star Traders.
I've seen Marvel pins at most gift shops, but I doubt you'll see sweatshirts/jackets in the parks right now since there hasn't been a movie released in several months. I agree that checking the Marvel store in Disney Springs would probably be your best bet (this store was much smaller than I expected it to be).
They do sell a shoulder Groot in the parks, my DD bought one!