I found 5XL, but no 6XL.
(1) Anthem Sports (I know nothing about the company):
Dalco D260 Umpire Shirt, Light Blue
The web page says the shirt is oversized to fit over a chest protector, fwiw.
(2) same deal, 5XL, from officialgear.com (I know nothing about the company):
https://www.officialgear.com/shop.as...d=t333&pid=130