The T-shirt, designed by comic artist and DePaul University associate professor Scott Roberts, is available in "Dadly" yellow and "Octo" (sadly, not Walter) white. You can pre-order it now for $14.99 for one week only.
"It's a hard life for an Octopus, especially one with so many secrets," the site's description reads. "Sometimes it's good to look into a little side-work for some additional income."
Update: Yes, the shirt is now available in "Walter" white.