Hello! American popping in here. Hope it's okay.
Just in case our silly airports deem cereal a threat and you can't bring it in, or if you want to buy some while you're here, the US Weetabix website lists several locations based on WDW's zip code.
Put 32830 into
http://www.weetabixusa.com/wheretobuy.php
Most well-stocked stores should have some type of muesli, usually in the healthy/natural section. I know because it's a favorite breakfast of mine.
Edit to add: I've heard the Publix near WDW is good about stocking items on request. You could try calling the store a few weeks in advance to make sure they have it.