If it's official MO WILLEMS STUFF, it's here.
Note: This is not a merchant site. These links lead to various vendors and the actual producers of the Mo Stuff.
Please check the stock of your favorite local shop first.
Most MO STUFF can be found on-line at MO WILLEMS SHOP at the ERIC CARLE MUSEUM GIFT SHOP, including Carle Museum exclusives and a rotating stock of signed books.
Another site with lots of stuff is the MO WILLEMS SHOP at DEMCO and OUT OF PRINT.