Hi,
I am having the same challenge. I want the same style shirts for my 2 ring bearers.
Connie'sKids.com has some ivory short sleeve button ups - the sizes are limited, but is worth a shot. I placed an order I need a size 5/6 and a size 12. She was out of the 5/6 and she informed me the vendor is no longer making the shirt.
As a back up I found off white polo shirts at GapKids that I will use for the boys
Shop clothes for women, men, maternity, baby, and kids | Gap
They're not my first choice, but I think it will work fine for my beach wedding...in the meantime still searching for the short-sleeve button up camp shirt in ivory